I'm not an advocate of things that prevent people from hurting themselves. Others, sure. It's just not my place to tell people not to hurt themselves, provided they understand the risks and consequences.
You cannot assume that people understand the risks and consequences. Ever. It's as much a flawed, ideal situation as assuming the government knows what's best for every individual.
People can be manipulated from birth. Hell, from
before birth. They are imperfect, not always well-educated, and not always entirely rational. None of these things mean that they deserve to be tricked into harming themselves through misinformation and advertisements designed to take advantage of them. Companies have learned to be very, very good at this, and your argument kind of falls apart when you realize that they are good at creating demand, not only supply, and that people aren't self-created wonderbeings capable of withstanding any amount of manipulation thrown at them, and if, for example, I am susceptible to something like pseudoscientific hoax medicine, that doesn't mean I deserve to die or be harmed by it.
There's also the simple fact that, indirectly, people harming themselves
causes harm to society. Mutual protection is one of the common goals of any society, and sometimes that means telling them not to do something that, for whatever reason, they don't realize they shouldn't do. The harm it does to society is clear enough: It incentivizes those who would manipulate others for profit, and
lots of mutually-shared cost go into the raising, support, health, and well-being of all individuals. In stark economic terms, society allowing someone to destroy themselves is ruining their own investment. A lot of effort, shared by family, friends, government, and society at large goes toward an individual throughout that individual's life, so wasting that is... well, a waste, and the health problems caused by self-destructive behavior also have an impact on society.
A guy who needs medical treatment because he got into a car wreck by acting like a fool, or got addicted to heroin, or has been smoking his entire life, has a negative impact on society, because he is no longer productive and there is a cost to treating that person. There is negative economic value to a person being self-destructive, and not only to that person. So even if you think that someone who acts self-destructive somehow deserves it (despite the clear and present influences throughout that person's life and the fact that you can't count on everyone or even anyone to always be well-informed and rational and perfect), that behavior still impacts the rest of us in a large variety of ways. Even if you don't give a damn at all about someone who uses heroin and dies of an overdose, that behavior is still seriously economically disadvantageous to the rest of us.
Think of all the effort that goes into feeding, clothing, educating, and otherwise supporting a person up through, for instance, their 18th birthday. Now imagine that person going out and dying due to some horrible decision that regulations are there to prevent. That's a lot of investment wasted, and in the case of the use of harmful products, that's a lot of profit going toward those who make a living effectively being parasites on the rest of us by selling things they know are dangerous. Incentivizing parasitic and self-destructive behavior are not good for society even if you
disregard the argument that protecting the weak is ethically responsible behavior in itself. You might say that you're only protecting them from themselves and that that's silly, but that isn't even true; even if someone engages in self-destructive behavior due to their own cognitive and behavior patterns, people are not born with those, and they can be manipulated, for the same reasons we don't allow the marketing of alcohol and tobacco to minors.