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Those are regulations upon the automobile industry.
They can make seatbelt installation mandatory, but forcing people to wear them is a violation of personal freedom.
It is what it is. Motorcycle helmets are exactly the same. Your state may require them, my state has declared helmet laws unconstitutional. This is from New Hampshire, the only state where seat belts are not required:
[New Hampshire has] "a strong emphasis on individual liberty and a reluctance to infringe on personal freedoms, rooted in their state motto, “Live Free or Die”. Many residents and lawmakers believe the government should not dictate personal choices, such as whether or not to wear a seat belt. "