Why aren't cigarettes illegal?

Many have said because it brings in a huge income tax and banning it would disrupt the right of the people. But that could go for any drug. I mean, cigarettes have Carbon Monoxide, Tar, Benzyne and other goodies in them; just as meth has drain cleaner and brake fluid. They're both terrible for your body. Furthermore, some state that cigarettes don't alter your behavior- alcohol is legal and it does. It just doesn't make any sense to me. Secondhand smoke damages children's lungs because their trashy parents won't respect them enough to go outside and do it. Even if they go outside then the neighbors are getting poisoned by it. I just don't seem to grasp the concept of why it is legal..