Maybe I haven't been following the calls for torture investigations closely enough, but it seems to me that there need to be two things that I'm not seeing for me to be convinced that the people issuing such calls are sincere about the issue and not just pursuing a politically-motivated witch hunt.
1. Speaker of the House Nancy Pelosi, Senate Majority Leader Harry Reid, and a number of other congressional Democrats were involved in discussions with President Bush and other administration officials when all this was actually going on, and they seem to have given their approval of whatever actually took place with official sanction. Or at least they voiced no objections. That's what I keep hearing. But I have heard very little about anyone seriously suggesting that they be investigated. The only reason I can think of for that is that they're Democrats. Someone with more information than I have should feel free to correct me on this if I've got the facts wrong, but it's very hard to see this as a movement to correct for mistaken policies and hold those responsible accountable unless all who were responsible are going to be investigated.
2. As far as I've been able to discern, the U.S. military has long used techniques like waterboarding in training their special forces to be able to withstand harsh interrogation techniques. My understanding is that they train them in techniques that are uncontroversially torture. Yet President Obama continues President Bush's claim that the U.S. doesn't torture. Those who accept it from Obama but didn't from Bush need to account for this, and if they think these procedures are immoral in principle then they ought to be consistent and issue a call to hold accountable those responsible for torturing our own troops, including any at high levels who knew about this and allowed it. (I suspect that would be all the presidents for at least as far back as Jimmy Carter, the earliest president still alive.) Again, it's possible that I don't have all the facts on this, and I'd be happy to receive corrections on this, particularly if you can back it up with sources I'd be likely to trust. But what I read of the very memos that everyone's getting all excited about now (even though they say almost nothing that we didn't already know) seems to confirm that this has been going on with our own troops.
I don't think this shows us one way or the other whether these policies are legal, morally justifiable, or worth pursuing an investigation about (and I see those as three somewhat independent issues). I actually think those issues are more complex and difficult to navigate than either side wants to acknowledge. See my 2004 post and then my 2007 pair on the moral and linguistic issues. (I can't say that I'd agree with everything in those posts now, though.)
But it doesn't seem to me that most of the people who are actually raising a big stink about this are doing so for consistent, principled reasons unless they're willing to apply it to the above two cases. (That doesn't mean they're all hypocrites, because they might not see the inconsistency and might be willing to adjust their behavior if they did see it, or perhaps they have arguments for differential treatment of the different cases, although I'm not sure what those would be.)