From what I've been told and have read, the fines for not wearing a seatbelt are less about preventing people from making a potentially life threatening decision and more about preventing people from making a really bloody mess that tax payers then have to pay to clean up. A dead body strapped inside of a car is a lot easier to clean up than a dead body thrown 60 feet down the road and torn into who knows how many pieces in the process. Not to mention that what is the equivalent to a human cannonball can cause further damage to property and people who are unfortunate enough to be in the way.
Thinking of the fine as "Increasing Cost to Society" instead of "Preventing you from endangering yourself" makes it more agreeable to me - as I think we agree on the stance that laws that prevent people from doing stupid things to themselves aren't good laws. Laws that reduce wasteful costs to society so that funding can be put into less wasteful things, in my opinion at least, are good laws though.
Since most laws against preventing people from doing stupid things to themselves saves wasteful spending - I agree with them within that framing of the picture.
You are forcing folks to internalize an externality. Same with carbon cap & trade schemes. Same with helmet laws. Same with toll roads. Etc...
I disagree: I am strongly opposed to force by violence (laws) to prevent people from doing stupid things to themselves. In this sense, I do not necessarily consider saving lives a good thing (of course, there do exist lots of cases where I do).
P.S.: I strongly recommend wearing seatbelts, but am opposed to seatbelt laws.