I don't think smoking is an actual cause of death. It's a risk factor for developing a disease. Lung cancer for example. I mean, if someone died of complications from diabetes, would we say they died because of eating sugar? Even people who have never smoked get many of the same kind of diseases that are common amongst smokers, albeit in smaller numbers. So it just doesn't make any
sense to automatically assume that smoking was the cause of death. It makes as much
sense as saying "not smoking" was the cause of death.
My point is that smoking is merely a behaviour. It can not logically be applied as a cause of death for a death certificate. Maybe the only exception might be if a non-smoker smoked one cigarette and immediately dropped dead, and the post-mortem found no cause for the sudden death. Somehow I doubt that's ever happened.