Ashcroft was granted the honour of having this tactic named after him after he argued pouring water down someone's throat was a legitimate interrogation tactic that should be enshrined in law, and that forcing water down someone's throat should be an international crime punishable by a decade's hard labour. 
And what do you know, we're back to torture again. Even the same form: waterboarding. Marc Thiessen would like us to know that comparing Americans to the Spanish Inquisition is ludicrous and unfair, because whilst both groups tied down their prisoners, prised their jaws apart, and poured water into their mouths to simulate drowning, the Inquisition tended to use less comfortable restraints.
I always wondered whether there existed another term for Ashcroft's Fallacy out there. Turns out there kind of is. The Rules of Wingnut Debating School  (t/h to Lemieux once more) describe this as "War on Analogy. Actually, I'd argue analogy goes out of the window when one compares the same action performed by two different people or groups. You'd need "War on Direct Comparison" for that. Moreover, there's another difference in that "WoA" attempts to argue that no comparison can be drawn at all, whereas Ashcroft's Fallacy suggests that the little differences mean that any sensible comparison should reinforce the massive difference in how the two are viewed. It's not "It makes no sense to compare Iraq and Vietnam", it's "If you look at Vietnam and Iraq, you'll notice that we have more tanks in Iraq, and that means we are almost certainly going to win!". Of course, the counter here is so plausible and depressingly ironic - that I'm picking up on irrelevant differences to try and prove AF and WoA are qualitatively different - I'm probably going to have to admit defeat and go back to never having coined any useful phrase ever (outside of maths, obviously).
Anyway, after knocking aside the sheer idiocy of Thiessen's obvious logical mistake, Yglesias wonders aloud:
[Why do] these kind of comparisons to the Spanish Inquisition and the Khmer Rouge and the Korean War-era People’s Liberal Army seem to bother torture advocates so much[?] The basic point made by torture advocates (when they’re not quibbling about whether or not you should call techniques poached from a torture resistance manual “torture”) is that the problem with liberals is that we’re not sufficiently willing to engage in brutal treatment of prisoners in order to compel their cooperation. But do you know who really didn’t shy away from brutal treatment of prisoners? The Spanish Inquisition! The Khmer Rouge! These are people who knew how to get the job done...There is an obvious answer to that question, of course: those that advocate torture whilst becoming outraged at comparisons to torturous regimes are that most dangerous of animal, those who are convinced that the ends justify the means and don't ever stop to realise that their ends are just as subjective as everyone else's. I'm not sure it would ever occur to Thiessen that there is no actual distinction between an American wanting to torture enemies of America to keep the country safe and a Cambodian wanting to torture enemies of Cambodia to keep the country safe. He presumably recognises, as must we, that the method by which enemies are determined is very different in the two cases, but it seems that this realisation is too often conflated with a belief that what is and isn't immoral treatment should depend on the accuracy of one's target selection.
Of course, I know little about Thiessen, so I'm hesitant to attribute arguments to him specifically. What we do know is that he believes that people declared enemies by the state should be tortured without anyone actually ever proving that the state-given label actually applies, because the benefit of protecting Americans outweighs the need to ensure that those foreigners they torture were ever actually guilty of the crimes for which they've been accused. In this sense alone, his ends: protect his country and its citizens from those who seek to destroy her, are a subset of those of the Khmer Rouge. Thiessen labels these people who can't be afforded due process "terrorists", the Khmer Rouge called them "intellectuals", or "subversives" (I don't know whether the Khmer Rouge avoided trials or simply rigged them, but the difference is arguably semantic when discussing the end result), but that is irrelevant in this context. What matters is that there are three separate issues that those who object to such distasteful comparisons appear to conflate: means, ends, and targets (as mentioned above).
Clearly, there is an overlap in ends: the desire to keep one's country safe. There is also just as clearly an overlapping in means. Both the "we need our enhanced interrogation techniques" crowd and the Khmer Rouge made use of waterboarding. The logical trap into which that former group has fallen into is therefore obvious. They believe the ends justify the means in this case, but don't believe almost the exact same end justifies almost the exact same means in the case of the Khmer Rouge. Aside from the fairly plausible possibility that nationalism is playing a role, I think the reason they don't believe that is that they know the Khmer Rouge used much worse means in addition, and had much worse ends in addition.
In effect, this argument boils down to "water-boarding is OK so long as it's the worst thing you do". The degree of illogical craziness there should be plain for all to see. Waterboarding doesn't become less reasonable if you're also executing people. Similarly, we have that wanting to protect your country is a reasonable goal so long as you have no other, less palatable aims in addition.
But does the Khmer Rouge's aim of a secure, peaceful country become any less reasonable because they also believed they were better off without the educated, or religious minorities (who they then killed en masse, which again is a question of means, and naturally a horrible and despicable one)? I would say no. I would say that attempting to remove those elements, quite aside from the moral issues (and I'm in no sense suggesting that's an easy thing to do), was clearly counterproductive to the stated ends, but just as one cannot say "Waterboarding is OK so long as it's our worst practice", neither can one argue "Wanting a peaceful, safe society is OK unless we also have other, far less palatable aims as well."
There are, of course, significant dangers with losing the woods for the trees when you set off on this sort of academic exercise. Whilst Thiessen's position is wrong, he is at least focusing on a specific set of crimes to be fought (again, this is the target aspect mentioned above). The Khmer Rouge dealt in the destruction of types of people. And even if we assume that Thiessen (or anyone else advocating a similar line) is just as wingnutaphilic across the board, then we're still only dealing with someone who is liable to believe the educated should be ignored and Muslims harassed at airports, rather than wanting them all exterminated. You can't complain about people realising how small certain gaps can be and then fail to notice how wide some others are.
Yglesias might argue that the intent is not to tar Thiessen with the more onerous aspects of the Khmer Rouge (indeed, the fact he claims not to understand why the comparison upsets people implies he is trying to separate the waterboarding from the more general context). If that's true, though, what is the relevance of the comparison at all? Stripping all that context away makes "The Khmer Rouge used waterboarding" no more compelling an argument than "Hitler was a vegetarian", the chief difference being the comparative difficulty in considering one method of oppression apart from many others compared to considering someone's eating habits apart from their crimes against humanity.
In short, it appears to me that either 1) Thiessen et al are right to be offended by the comparison, because it carries with it the implication that they are or desire to be like the Khmer Rouge in other ways beyond the one under discussion, or 2) no such implication comes alongside the comparison, in which case the comparison itself isn't actually at all useful. Finding a hideously oppressive vicious regime that ruled waterboarding a step too far, and you might have something (though even then it would need to be sensible, try telling a steak-lover that even Hitler didn't stoop so low as to eat beef and see how quickly you get your veggie head kicked in).
None of this speaks against a more general point of Yglesias', though. Those that advocate waterboarding tend to argue the ends justify the means whilst knowing full well that they don't believe that across the board, and until they admit that and explain why these ends justify these means, they're simply insisting simplicity exists where it does not (a typical right wing trick). There is also one additional wrinkle to the Khmer Rouge consideration: it leads to a potentially interesting thought experiment. Consider any given victim of the Khmer Rouge. Intellectual, woman, child, religious minority, whomever. Now stick him or her in a room with Thiessen, and tell him that he or she is a Muslim extremist who knows where a time bomb has been planted in New York City. Then hand Thiessen six foot of rope and a bucket of water, and ask what he thinks needs to be done.
We might find that the comparison isn't quite so ludicrous, after all...
 Actually, I'm being unfair on two counts here. Firstly, Ashcroft didn't argue that the latter act should be punishable by international law; I doubt he believes in the concept of international law at all. He did though argue (albeit implicitly) that in situations in which the international community had served up said sentence, those that were so sentenced got what they deserved.
The other way in which I'm being unfair is that the argument that persuaded me to name Ashcroft's Fallacy after the gentlemen in question was actually worse than Ashcroft's Fallacy itself. His was a specific sub-division by which even the difference between the two acts doesn't have to be strictly positive, so long as their descriptions vary. In this way, Ashcroft could argue with a straight face (though he didn't; he was pretty mad at the time) that forcing water into someone is worse than pouring it into them against their will.
 Just for the record, whilst I believe there are a great number of commonly-held right wing concepts that cannot easily be defended through means other than those listed in Wingnut Debating School, we would do well to remember that those particular debating tactics are common on the left and in the centre too, it is simply that those of us on the left are liable to spend our time mainly arguing with the right, and thus see most argument tactics, both good and bad, more times coming from right.