Hypocrisy has the advantage of offering a little chance of leverage, and if the plainest tyranny is presented as acceptable, it's necessary to build up a moral system from scratch. I guess it is, anyway.
The Overton Window is a concept from political theory-- policy isn't decided from among all possibilities, it's decided from what people consider to be worth imagining. A lot of work going into shifting the window-- this can be seen recently in regards to gay marriage. To my mind, S3801 is giving the window a hard yank in a bad direction.
The other point: this doesn't fit with the usual narrative of partisan politics. If George Bush tried to expand the power of the presidency, that's ordinary status-seeking.
However, as you may have heard, Obama is a Democrat with Republicans in congress as a bunch of very pointy stones in his shoes, but the supporters of that bill are 9 Republicans and 1 Independent.
Here's an interesting theory:
I've heard someone posit that McCain and Lieberman intend to put this bill as a rider on the health insurance reform reconciliation bill or some other bill they know Obama won't dare veto. I haven't found any confirmation for this, and I don't know why they'd bother. Obama has already announced his plans to hold between twenty and sixty of the current Guantanamo Bay prisoners forever without trial, so that part of the bill obviously wouldn't bother him at all. And as for banning trials of any such prisoners in the future... well, all signs currently point to Obama backing down on Holder's attempt to try Khalid Sheik Mohammed in civilian courts.
At present, I can honestly see Obama signing this bill and praising it as a regrettable but necessary tool in the ongoing (and eternal) war on terrorism.