People who are very concerned with their health tend to avoid bars on principle. That is their perogative and I respect that.
Sitting in a bar drinking alcohol with other folks who are also drinking alcohol is an inherently unhealthy activity. But for many of us, it is an enjoyable activity where the blessings outweigh the risks.
These people supporting the ban have no intentions of patronizing bars. They have the mindset that "there is a rare but distinct possiblity that one day I might walk into a bar and it would be nice to not smell smoke."