How do you know? Smoking is antithetical to health/life. If a person is not free to smoke, then you're saying that you have greater claim to their body/life than they do, which cannot be universalized. As long as they are not forcing others to smoke, they can smoke and the behavior is amoral despite being antithetical to life/health. The measure of morality is whether it is binding upon another or not and if so, was consent secured?
RE: Existentialism: The Philosophy of Freedom and Personal Responsibility