That same question applies to helmet laws, transfats, and other various lifestyle laws. When I have pushed people on the issue, they've argued that my medical costs will cause insurance to go up for everyone, and so the more that people are wearing their seatbelts (and helmets), the more affordable insurance will be fore everyone.

American is tendency toward totalitarianism. Not scary dictatorship-like, but the tendency to believe that the government is the solution to all our problems and should be involved in every aspect of our lives: what is healthy to eat, do, buy, wear, etc. Scientists tell us what is good, and government enacts laws to require it. You might take a look or listen to "Liberal Fascism" by Jonah Goldberg. He discusses how various invasive policies that have been around in America for decades (like the "war on drugs") are on the part toward fascism.

I'm more of a libertarian; if your behavior harms another person, then you will be punished, but otherwise, you should be allowed to make your own choices. As for seatbelts, I think insurance companies could have a policy of not paying your medical bills if you chose to drive without a seatbelt, and then wearing a seatbelt is your choice, and your mistakes will harm no one else.