I don't like ex post facto laws either but they do creep into the codes through the back door.
BTW you really need to look at each state law before you make blanket statements about retrofitting seatbelts. In Florida the only exception in 316.614 F.S. is for pickup trucks
"(b) The number of front seat passengers of a pickup truck required to wear a safety belt pursuant to this section shall not exceed the number of safety belts which were installed in the front seat of such pickup truck by the manufacturer."
Autos are not exempted.
