Do you really think the danger of you being flung out into incoming traffic is the driving motivation & justification for seatbelts? Why not allow people to not wear them in the backseat then?
I'm not saying that at all, just that you could make an argument for seat belt laws that doesn't involve minimizing/avoiding unnecessary medical costs.
Eh, it's not really a persuasive argument though. Because it takes up police resources and countless hours of citizen and legal time to enforce, not to mention the enforcement on the completely negligible backseat passengers under this framework....all to prevent flailing bodies from disrupting traffic?