People have a right to make stupid decisions, even ones that would be dangerous to themselves.

We know this is the case because fast food isn't illegal - it's just that people (in this thread) are too stupid to see the contradictions in their own logic. Seatbelts are just a really oddly specific example of people choosing to police other people's stupid decisions, for some reason.

it’s appropriately forcing someone who can’t be an adult to act like one.

I don't think I could have worded this in a more infantilizing manner if I tried, so thanks for proving my point I guess?

in reality this kind of law is very effective at making more mindful of their safety, even if idiots among them do it begrudgingly.

There are other ways to incentivize this behavior that doesn't involve becoming an infantilizing nanny state. Significantly higher insurance premiums for people who choose not to buckle up, for example.

Cigarettes are another great example - they're not illegal, but the government gives people financial incentives not to use them in the form of sin taxes.

There are many ways to disincentivize stupid behavior while still respecting the right of people to make those stupid choices.