When you are in a collision with out a seat belt more often than not your body becomes a 100 pound plus projectile going at non trivial speed that could easily kill some one.
Consequently even libertarians might be for seat belt laws (unless of course you are on private roads).
I'm not sure of the accuracy, but I've also heard it argued that you're much more likely to regain control of a vehicle after an evasive maneuver, since you will stay in your seat.
It's not. A federal/state law is a law. You are compelled to do something that may be against your wishes. Whether or not it is a good thing is neither here nor there! To spin the debate, apply the same for drug laws, or substance control.
