I understand your reasoning, but I think I am for seat belt laws for this reason.
If there are no seat belt laws and for some reason you are in an accident with another car, and it is your fault. If the other person is not wearing a seat belt and dies, I would be worried about being sued or going to jail for manslaughter.
By having the laws on the books, then I wouldn't be worried about that if the other passenger was wearing their belt.
That makes as much sense as that disclaimer on the new birth control pill that cautions users that taking the pill will not prevent the risk of catching AIDS or other STDs.