> The argument for requiring a seat belt isn't about protecting people from their own choices. It's because it's very easy to lose control when things get exciting if you're not belted into the seat. Now you're throwing around a two ton projectile and everyone else's lives are at risk.

This can't be the complete argument, because then it would only apply to drivers. If you're not wearing a seat belt and you're in an accident, you might become a projectile which can cause harm to others in the same vehicle.