If smoking reduced total health expenditure (because those who die early from smoking-related illnesses avoid the costliest years of their lives), would you advocate for the government to encourage smoking?
I considered multiple facets when making this decision: 1) people are harmed by it, 2) even though people willfully start doing it, they can't always willfully stop doing it, 3) it negatively impacts them and their family's health/finances, 4) it negatively impacts society in a variety of ways.
I believe that impacts to society is what gives society (read: government) the right or obligation to intervene. Taxes are an effective mechanism of softly encouraging good behavior and they have much less risk of unintended consequences than total bans do.
Not only that, but cigarette companies target youths with an addictive substance that people eventually want to stop using but have a very difficult time doing so.