Unfortunately, that same argument could be used to make alcohol and red meat illegal, to prohibit sky-diving or baseball, and essentially to allow the government to determine the acceptable level of risk in every area of our lives.
Direct harm to others (and by that I explicitly mean more than "harming yourself always harms others") is necessary to justify making something illegal. The default position is liberty; taking away liberty requires justification, and that justification has to be more than an adult being harmed by their own informed choices.
A society where the government can limit your behavior "for your own good" is no longer a liberal society.
Um. Your slippery slope is well noted. However, there are lots and lots of things that we do for the protection of society instead of the protection of just the individual. Prohibiting drunk driving, for example -- the drunk isn't the one who gets hurt, other people and property are. Prohibiting smoking in public buildings isn't done for the safety of the smoker, it's done for the benefit of the non-smokers around them. Prohibiting illegal drug use isn't just done because of the impact upon the individual using the drugs, but also because of the impact that user has upon the people and economy around them. And then there are things like the banning of trans-fat that NYC and California has done "for the benefit of the public". Yes, our government acts (whether I agree with those actions or not -- there are many I don't) for the benefit of society as a whole, not just the interests of the individual. And that's how it's supposed to be.
Risks are weighed, and quantified, and measured. The probability of an unbelted person being in a vehicular accident, combined with the severity of the possible end result of being unbelted in that vehicular accident, is far far greater risk than the probability of someone eating a single steak, and the severity of the end result of eating that single steak. Period.
Your analogy about unhealthy eating is ridiculously inane. You need to realize that. Eating a single steak will not kill a person. A single car ride without a seat belt can not only kill that single person, but can also result in the deaths and hundreds of thousands of dollars worth of property damage.
It's called a risk management assessment. You might want to learn how to do one before you start throwing out even more ridiculous analogies and excuses as to why it's stupid for the gov't to have seat belt laws. I even linked you to a NHTSA report about said risk management assessment (they call it an economic impact study, by the way). Did you read it? I'm guessing not...