Answer

In principle, a state allows itself to forbid a person from causing harm to himself. That is true regarding cigarettes as well, except that there the damage is apparently not so great and direct, and such a demand may be too strong. Of course, one can argue about where exactly the line should be drawn.
The justification I can see for this is that if a person harms himself, then he and his family become a burden on society (medical care, social services, etc.). Therefore society says that it is not willing to allow this. And of course there is also the concern of harm to other people as well (if you don’t wear a seat belt and are injured, you may lose control of the car and others may be harmed).
That is the justification. As for the right to do this (and that is a different question), this is a matter of using resources that society places at our disposal (roads, streets, and even cars, and of course other social services — security, education, welfare, etc.). Therefore society can make the use of its resources conditional on various terms.
And beyond all that, if you accept the view that a person is also a limb of the public collective and not only an individual, then the collective can make decisions for him (just as it makes decisions for itself).