the Congress says so: "outlawing waterboarding rests with Congress."
So to hell with international law and every relevant historical opinion and precedent. According to the Spector doctrine, unless the moral cowards in the US Congress can pass a veto-proof, signing-statement-immune law explicity forbidding it, the president of the United States can use any form of "enhanced interrogation" technique he wants. This whole gang should be hung as war criminals.