If they don't have their own well being at heart, what guarantee is there they have their passengers well being considered?
Maybe because aside from the fact they smoke, they hold a Professional Licence, have a desire to do a good job, and also to reach each destination in one piece also? Just because you smoke (and yes, it is a terrible habit) it doesn't automatically preclude someone from wanting to live on a daily basis.
I agree that physiologically it would appear odd not to pass a candidate who doesn't smoke (all other factors being the same), but psychologically?! really? just because I smoke, doesn't automatically make me a danger in all other areas of my life.
F/o