Depends on which law book you read.
First: Have you ever read a warranty all the way to the end? Was the last sentence "You may have other rights which vary from state to state"?
Laws vary from state to state and from "Age" to "Age"
In Michigan for example. ALL front seat occupants must be properly "Restrained" with seat belts or a proper child restraint system depending on the age of the occupant.
Rear seat passengers are required to be properly restrained under the age of 16, 16 and over can choose to live or die should the vehicle crash. (And yes, I've seen more than one accident where seat bealth either made the difference, or would have had they been used, all, were, of course, fatals, the person WITH NO BELT died, (Save in one case where the person with no belt was thrown from the vehicle, thus loosing control, and the now human-less truck went on to kill two people, again, a seat belt would have saved TWO lives in that one)
Other states, laws vary
The law of physics that I kind of allueded to above, the one that says your chances of walking are EIGHT times better WITH a belt than without (Both of surviving and of surviving with little or no injury) That one is universial, it's in force EVERYWHERE and the penality for violation may well be death! (Or what's worse)
Several years ago I had a baby daughter, and needed a baby sitter (She's 27 now) Hired a teen who had recently been in an accident... Her friends (3 of them) laughed at her when she buckeled up. She had the good grace not to laugh at their funerals. They broke the law of physics, They paid the price.. She did not, She split her lip (that and some brusing was all she had, not even a trip to hospital was needed)
After that I saw a truck, rollover, 3 people, Mommie, Dead. Daddy Server injuries Both, no seat belt, Junior, in compliance with the law at that time belted.. He had minor injuries. Such as I might do shaving in the morning, from flying glass. Not even a band-aid was needed, just a few seconds of pressure.
I'm sold on belts