People have been using tobacco for many thousands of years. if they want to use it knowing full well the consequences, they should be able to. Unless we also ban things like skydiving, rock climbing, and fast cars and motorcycles, it makes no sense to me.

Why isn't prohibiting something known to cause harm a good thing? Plus, smoking doesn't just harm the individual doing it, its harm extends to those in the immediate (and sometimes not so immediate) vicinity, as well as the environment. There is literally zero good to gain from it.