I'm not sure exactly what the grandparent was trying to say, but seat belts became mandatory new car features in the US back in the 1960s. Airbags became mandatory new car features in the early 1990s.
On the other hand, I believe seat belt usage was still under 50% in the early 1990s [1], when the airbag legislation was being written. (I was born in 1970, and the first time I remember wearing a seatbelt was the summer of ... '85? Getting a ride from a high school friend to football practice every day quickly made me a convert. :) )
What I understood was that US regulations for airbags are intended for cars that have airbags only and no seat belts at all. And that those same requirements continue to be applied the airbags + seat belts vehicles we actually encounter.
(cynically, this seems entirely exactly like something Detroit would push for in order to disadvantage imports--so, although this sounds ridiculous, it rings as entirely admissible to me as "no duh" after reading that comment)
Light-duty trucks aren't considered cars in many states and so laws like seat belt requirements and emissions standards don't apply to them. I think it was just this past summer that seat belts became mandatory in trucks in Georgia. It wasn't until 2009 that light-duty trucks needed to be emissions tested and there's still all kinds of exclusions.
When I was a kid, in the 1970s, our car didn't yet have seat belts in the back, but in the front, they were required. I think when they became required in the back, it also became required to use if you had them, which obviously older cars didn't.
But having seat belts for decades and then not expecting people to use them, that's kinda silly.
I think "not expecting people to use them" is in accordance with actual facts, a substantial portion of drivers would flat-out refuse to wear seatbelts. (I feel as if this has changed in recent years, as those people age out, mellow, see reason, or die in car wrecks).
On the other hand, I believe seat belt usage was still under 50% in the early 1990s [1], when the airbag legislation was being written. (I was born in 1970, and the first time I remember wearing a seatbelt was the summer of ... '85? Getting a ride from a high school friend to football practice every day quickly made me a convert. :) )
[1] http://www.freep.com/story/money/cars/2015/07/30/michigan-se...