I genuinely despise smoking; if any of my mates smoke I ask them (politely) to not do so near me! However, as others have pointed out, banning smoking would open up a whole can of worms, and would cause more problems than it is worth. To be honest, I would be stunned if smoking caused more health problems than McD's and Burger King.
While I can understand the OP's argument that smoking damages the health of non-smokers, while fast food does not damage the health of healthy eaters, the recent banning of smoking in public buildings has done a lot to combat this. The only place where you can now breath secondary smoke is outside, and this is very probably no worse than the air which most people breathe in cities every day.