The truth is that there is a rogue presidency and there has been, since January, 2001 (earlier, if you count the stolen election). Certainly, everyone in Washington knows it, but no one dares to admit it. The bill legalizing torture merely enables Congress to pretend they still have some influence over an executive that from day one was governing, not as if they had a mandate, but as if Bush was a dictator. If, for some miracle, the bill didn't pass, every congress-critter knows Bush would keep on torturing.
Better to vote to pass and preserve the appearance of a working American government, the thinking goes. For the very thought that the US government is seriously broken - that the Executive is beyond the control of anyone and everyone in the world - is such a truly awesome and terrifying thought that it can never be publicly acknowledged. If ever it is, if the American crisis gets outed and Congress and the Supremes openly assert that the Executive has run completely amok and is beyond control, the world consequences are staggering. It is the stuff of doomsday novels.