> As long as a group within "everyone" (or multiple groups) decide that their support is required to pass the bill, they can suddenly demand concessions
Well, yes, but then everyone doesn't really want it, do they? Someone wants something else, and wants that something else enough that it is worth jeopardizing the supposedly universal goal for it.
If you've ever negotiated, I bet you've done the same thing of jeopardizing something you want in order to get something else you want. If you never do that, you'll make a lot of deals where you're riding the edge of just barely acceptable and the other person is taking advantage of you. But in this case, with a standalone law, doing it gets pretty rude and we'd be better off if nobody did it.
Well, yes, but then everyone doesn't really want it, do they? Someone wants something else, and wants that something else enough that it is worth jeopardizing the supposedly universal goal for it.