Legally an employee has the right of a smoke-free working environment. So the operator or owner can decide what is tolerated in their aircraft and it lies with them to choose a crew that wouldn't mind the smoke.
In some countries (e.g.South Africa) it is fobidden to smoke on any aircraft on their register - so basically part of their airlaw, which makes things quite clear and easy.
I have had collegues craving for a smoke on ferryflights and have noticed some becoming extremly irritable and agro, pushing up the throttles to Max cont. to get on the ground a few minutes faster - or lighting up a cigarette anyway without really caring if I like it or not - not a good habit and a demonstration of personal weakness, lack of respect and poor willpower.
In my personal observation, smoking is a stress factor and should simply be banned inside aircraft.