That is so stupid. It should be a law that you have to retrofit your vehicle with seatbelts.
Why, that is a personal choice. If someone doesn't want to wear or have one it will not kill you. Same as an alcoholic (non driver), smoker, sex addict and abortions. It effects the one making the choice. If you don't agree with it you don't have to smoke or drink. Become a priest. You can have as many babies as you want and you don't have to ride in the damn car.
Nanny state or not I believe the bottom line for most laws in the 20th and 21st century is the generation of revenue.