Now the enforcement issue is a whole other kettle of fish. I am strictly talking about if seat belt laws are legitimate, and the answer is yes.
Wether or not they are legit depends on perspective imho.
When it comes to enforcement they are legit because it is the law. Law should be enforced equally and IMHO the best way to insure their use is to incorperate technology in the car that requires their use.
Seems that would solve the entire issue eh?