[sybir]

I was under the impression that shops would not do it for liability reasons, regardless of paperwork, but owners were allowed to do so without restriction, as there aren't laws or vehicle codes requiring an airbag in a driven vehicle, just requirements for new cars to come with them.

You can legislate seatbelt use in a vehicle that came with one, but that law came into effect what, 30-40 years after all new cars had them federally mandated? You can still drive a 1929 model T with no seatbelts legally - at least in CA, it's every passenger with a seatbelt in their position in the vehicle that has to wear it. Bolt seats in the back of a van, or sit in the back of a truck with a shell on it, you don't need anything.

Note that I'm not saying it's wise not to wear a belt - just that there's no law pertaining to continued operation of supplemental restraint systems. If I get an airbag light, you can't force me to pay $2-5k for a new component or system (potentially) and I can address it as I want, just like you can't force me to correct a CEL in a specific way (smog laws aside).