Well, like I said before, I don't really care one way or the other. I'm just putting some reasons out there that are worth thinking about.
It's not about taking away people's personal freedoms to make bad decisions. It's about having simple rules that don't negatively affect anyone, in order to help people make good decisions.
With your food example, that's a completely different thing because people get pleasure out of eating shit that's bad for them. Eating cakes every day is horrible but there's a legitimate reason to have some at least once a year (birthdays). Just as an example. But maybe there are some things we can do to help people make good decisions that don't negatively affect them? Maybe creating a new version of Coke that tastes exactly the same but doesn't give you diabetes. Or offering kids in school a choice of lunches that aren't frozen/fried garbage.
Regarding the seat belt thing, consider this: "lap/shoulder belts, when used properly, reduce the risk of fatal injury to front seat passenger car occupants by 45 percent and the risk of moderate-to-critical injury by 50 percent. For light truck occupants, seat belts reduce the risk of fatal injury by 60 percent and moderate-to-critical injury by 65 percent"
http://www.nhtsa.gov/people/injury/airbags/Archive-04/PresBelt/america_seatbelt.html
If you could make a simple law that doesn't negatively affect anyone, but significantly reduces injury/death, why wouldn't you do it? I understand what you're saying about having the freedom to make bad decisions, and I agree, if I want to go basejumping, I dont want someone telling me that I can't. But on something that there is no legitimate argument against (like not wearing a seatbelt), why not?