Seat belts laws save tons of lives. Not wearing one subjects you to a fine, and there was plenty of uproar years ago as states started requiring you actually wear them. This was well after the federal government mandated their inclusion in vehicles.
You can look at seat belts from a simplistic point of view and conclude that not wearing one doesn't actually harm anyone. Not in the "moment", no. You can also look at it from a simplistic point of view and conclude that if anyone is harmed, it is the person who chose not to wear one.
The impetus behind pushing seat belt laws was that without forcing people to use them, they didn't help to save lives. When I (theoretically) get into an accident with someone and it is my fault, that sucks. I pay the increased premiums and move on. When I get in that same accident that is my fault and the other driver isn't wearing a seat belt and dies, I have committed manslaughter.
Our society, exceptions aside, generally want things to be safer even if it means a small inconvenience like buckling up. Or switching out a magazine 40% more often.