I acknowledge that I am not well qualified to judge this issue. I am not a lawyer, but I have wrestled with the law on it enough to know that it is far from simple and a matter of dispute among lawyers. As now seems to be mandatory on this and other issues, positions are stated with false certainty and with unyielding moral absolutism. It is necessary to read everything sceptically.
The earlier cases do not prove that waterboarding as practised during the Bush administration was illegal, only that waterboarding carried out in certain ways and under certain circumstances has been successfully prosecuted. The designers of the policy knew the law and manoeuvred--absurdly and offensively, perhaps, but they would not be the first lawyers to stoop to that--to stay within it. As for whether, regardless of domestic law, the international Convention Against Torture mandates prosecution, you have to understand the distinction between treaties that become the law of the land in and of themselves, and treaties that the US adopts, and in effect modifies, with a law of its own. The Convention Against Torture is of this second type. Arguably, therefore, relevant parts of the CAT are not enforceable in US courts.
Incidentally, there is further disagreement over whether the US government has discretion not to prosecute, even if it takes the view that a law has been broken. Some constitutional lawyers say it does not. The administration's promise not to prosecute interrogators implies that it thinks either the law was not broken, or else that it does have discretion not to prosecute.
If prosecutions were brought, could one count on getting convictions? Because of the deliberate imprecision of current law, the defence has a case to make, and a jury, reminded of what was at stake in the aftermath of 9/11, might be inclined to listen to it sympathetically. So one at least needs to ask, what would be gained by prosecuting these crimes and seeing the defendants acquitted? Surely this would undermine rather than affirm a US commitment never to use these methods. And I think the same goes for the suggestion being made that culprits up to and including George W. Bush should be prosecuted and if found guilty pardoned. I admit, when I first read that I thought, "Only in America". We stand on the principle that torture is a crime and will be prosecuted without fear or favour to the fullest extent of the law (with pardons to follow). How's that for a clear message? But at least the rule of law has been upheld, you might reply. Well, as I have mentioned, the rule of law will also be upheld, according to one school of thought, if the Attorney General exercises his discretion not to prosecute.
As this leader in the FT notes, what matters most here is not to put George W. Bush and his team in jail, or to try them and then pardon them. It is to guard against such measures being used again. That is a political as much as a legal project--it requires the building of a moral consensus, the changing of many American hearts of minds--and in my view it is best advanced not by prosecutions but by the "look forward" approach Obama first said he wanted to follow.