In most states, as far as I know, it's illegal not to wear a seatbelt. So from the point of view of the law, it's not optional. Pragmatically, it's somewhat optional because it's unlikely that violations of the law are detected and prosecuted.

From a "should this actually be the law" point of view: I don't know.

I think that in US society, our championing the rights of the individual has led to a failure to protect the individual from themselves.