@"I'm not sure there is such a thing as "natural rights" at all, since rights only exist in relation to other people and are respected or not by social convention and agreement only."
There is a concept of natural justice in some legal jurisdictions (perhaps in most, but in some the have more weight). You can argue your actions were justice based on nature, whereas the law didn't specify whether they were allowed. (Perhaps you can break the law, but then claim by natural justice that your actuons were right and the law was wrong)
I think this is related to your natural rights.
To those still pointlessly arguing, there is a reason these laws exist. It is a historical issue, seatbelts were first invented and proven beneficial, but people had no havit of using them and placed no demands on manufacturers to add them as standard.
First the state mandated them as standard, then when use didn't increase they mandated everyone break their habits and get ised to wearing seatbelts.
It becomes less important now, the culture has changed, i doubt anybody would buy a car without a seat belt; though some may still choose to not wear one (or forget, we're not perfect, though i'm sure some manufacturers now add a reminder feature which helps force the decision)
This is an example of regulation, which successfully saved lives where the market was selling a dangerous product. And it is the role of the state to take these kinds of actions. (My story is entirely made up, but i've no doubt you can find out the history, and the arguements that went with it, if you bother to look them up)
Likewise the state should act to reduce smoking rates (and has, many times over, from banning advertising, to taxation, to anti-smoking campaign ads) this is a different more subtle form of control. But it is the same, an attrmpt to change our habits for the better.
The same will be done for unhealthy food and diabetes medication, though the food industry may be a little harder to fight... They will not simply ban the purchase of unhealthy food; that would be incredibly unpopular - so no slippery-ness on this slope, as far as i can see.