As I said earlier in this thread ...Laws regulating personal conduct can be justified on three grounds, one of them being The Harm Principle. Which states that laws are justified if they prevent individuals from causing harm to others. In other words, the second hand smoke argument.
So for example if you were to inject the drug (nicotine) into your system or say wear one of the stop smoking patches, no problem since nicotine is currently legal. But since the act of smoking has been determined to be harmful to others around the smoker, particularly in a closed environment like in a building, it can be made illegal. In fact, I believe that in California it is illegal to smoke in your home or your car if there are children there. Good luck enforcing that, particularly the home part, but I believe they did make that law.