They are also using current statistics to come up with their conclusions. Societal disdain, being told the medical reasons, price and inconvenience have made many, many people quit smoking. I would expect that a few of those things make no difference to the mentally ill.
I just looked and in 1965 47% of the US population smoked. So if that was average everywhere and now ONLY 40% of the mentally ill smoke, on average, 7% have been affected by price, fear and peer pressure.