This argument falls apart for countries with socialized healthcare.

As long as all people are paying for your dumb decisions, it is reasonable to expect the government to reduce the frequency of dumb decisions by adequate means.

I notice that these sorts of justifications for increased paternalism as a consequence of socialized services come up in public discourse all the time but never seems to be mentioned by advocates when proposing these socialized systems. It should be mentioned up front as a significant cost as part of the package, it comes with strings attached like the government telling you how to live your life. Interesting that people don't seem to want to mention that up front.

Enforcing a healthy diet and exercise would have a vastly larger impact than any seatbelt laws in terms of reducing health care costs. Seatbelts and smoking always seem to be about as far as the advocates are willing to go though.