Why the government makes you wear a seatbelt -
People are stupid and will get themselves killed if no-one stops them. We don't like people dying, even if they are idiots.
Getting into all these arguments about unrestrained bodies popping through windshields are somewhat irrelevant, in my opinion. As has been pretty damn established, and I think not even argued against by the OP, wearing a seatbelt will save your life in the event of a crash, which raises the question of why the hell wouldn't you want to wear one?
And, yet, there are people that are either stupid, lazy or ball-poppingly arrogant ("Crash? Me? Nah mate, I'm the best fookin' driver in the world! Look. I'll show you, see how fast I can go!") enough not to wear one, so, alas, we have to force them.
Yes, in a perfect world, we would force things on no-one, we would all be our own boss and everything would be great.
Ain't going to happen though, is it?
As for your statement of 'But where will they stop?'... Well, you have a vague point, but you're taking things to a ridiculous extreme here.