First of all, what did you mean by natural selection?
About the fast food: Many people like to look at everything from a completely detached, individual viewpoint. But we live on a planet where we cannot just ignore each other. This is why we have traffic laws. We don't just say "stay away from the road and you'll be fine", we try to make a good, efficient, working system with rules.
Tobacco, alcohol, fast food, drugs, etc. is harmful to society. It's harmful to people because it's unhealthy, and when a lot of people are unhealthy, everyone else have to pay more taxes because they work less because of their health, and the healthcare system becomes more expensive too because people are using it more.
Another point about the fast food (etc.). We want our society to be as good as possible for everyone to live in. Having a poor health is not good. The people who start smoking, doing drugs, eating candy etc. hardly understand the consequences, which is why laws forbid drugs. But corporations are too busy making money of people's health, so sadly we're still wasting Earth's resources on producing all the other crap. I also care about Earth's resources, because they're finite and should be put to the best possible use. The best use is not clogging people's arteries or destroying their lungs, giving them cancer and killing them.
Now, the third point is more about the corporations. They don't even pay taxes, but they sell a lot of useless stuff harming our society. They use transfer pricing. In my opinion, these greedy people can kindly ... off.
Do you understand why I don't just ignore them now?