There's this guy I know. He's allergic to pollen and fronds. He'll never be a florist.
The difference is that almost
everyone is "allergic" to tobacco smoke. After long time exposure to tobacco smoke any given human being has a high probability of developing either lung cancer or emphysema.
What you're saying is that anyone who works in a bar should accept the risk of developing a deadly or chronic disease.
I believe all work environments should leave the worker with a reasonable chance of avoiding permanent injury or death through due diligence. The reason this should be
legislated (and not just a common understanding) is that there are poor people who will do any job, and employers should not be able to exploit their situation. Rather, the employer should be forced to alter the work environment.
There will of course always be hazardous work environments, but as premised above the worker should always be left with a reasonable chance of avoiding the hazards.
An example of a hazardous environment with avoidable dangers is a meat-cutting factory, where there are machines that when mishandled can cause injury or death. However, through a proper work tempo and observance of routines the meat-cutter can avoid dying or permanently injuring himself with the machines.
Spending the bulk of your time in a tobacco smoke-ridden bar is very different. Here, the worker has no chance of avoiding the health hazard. There are no "routines" to observe, save leaving the work environment completely (and thus not doing your job anymore).
If you agree with the above premise of work environment safety, the only alternative to banning smoking is strictly legislating the work hours of bar workers.