Keeping flying bodies to a minimum has never been a justification for seatbelt laws. And it is not a significant risk factor in an accident (you being harmed by a body flying out of another car). The rationale is forcing people to do the commonsense thing to reduce the risk to themselves.
The point I keep making that so many don't seem to understand is, once the state begins to protect people from themselves, where does it end? I can see it now, public service announcement "Don't forget to brush your teeth before you go to bed tonight. It's not just a good idea, it's the law!"
In this case, since it is such a commonsense thing, and in many cases it has been a law for many years, it doesn't seem to generate any outrage. But it is a classic case of the Camel getting its nose under the tent. Soon you've got the whole damn thing in there. The precedent has been established. The state has the right to prevent you from taking risks to your life that, in its infinite wisdom, it has deemed unnecessary. So what's next?
Motorcycle helmet laws are another example. Though wearing them is a good idea, that doesn't mean it should be the law. And if the rationale is saving the state money, how many millions of dollars could be saved if all automobile drivers had to wear a helmet? Ever see a race car driver without one? Even as much safer as cars have gotten with airbags etc., many lives could be saved if everyone donned a helmet before driving anywhere. Hey, lets make it a law!
Here in California, I can't legally ride a motorcycle 20 MPH without a helmet. But I can fly down a local mountain pass on my bicycle at over 50 MPH without one.
Laws are to protect people from each other, not to protect people from themselves.