These are just my personal thoughts, and I'm just another wannabe, not an expert in Aviation medical:
If you think about it, airlines like BA can justify not employing smokers. Because many airlines operate no-smoking flights. And obviously this includes the crew. Smoker's anxiety builds up when they haven't had a fag for ages, and no airline want high levels of anxiety in the cockpit during a Long-haul flight. This could cause safety issues.
Again, let me reiterate that these are MY thoughts.
Cheers
BTW