and I can't say I have any real basis for thinking this is what's behind it: in the Senate, if you vote with the majority on passage of a bill, you're entitled to call it back up for reconsideration, I believe within a certain time frame after passage.

Sometimes you'll see a Senator who's opposed to a bill or a particular provision in a bill switch his or her vote in order to be able to do this.

The Democrats who voted to pass the bill may very well legitimately support it, for whatever reasons. But they may also be guaranteeing themselves the right to move for reconsideration if the bill comes back and they discover the offensive provisions haven't been removed.