What about the other prisoners and guards who don't smoke having smoke imposed on their lungs? Isn't that ridiculous? I'm suggesting we make this fairer, by not forcing inmates to breathe poisonous fumes in addition to serving their sentences. What makes prisons special compared to every other indoor public space (in most western countries) where it is banned - why are they exempt?
This is like saying "OMG, <thing that is regulated by the states> isn't illegal in the US". Prisons are run by the states. Jails are usually run by the counties. Banning tobacco across the board for prisoners and staff is common because it becomes one less potential avenue of illicit trade between the guards and prisoners. Just because there isn't a law against doesn't mean it's not usually prohibited in practice.
[1] https://en.wikipedia.org/wiki/Smoking_ban#Effects_of_prison_...