The justification given to ban smoking in public places is that secondhand smoke has been shown to be just as effective at causing cancer than inhaling smoke directly from a cigarette, and hence, that the smokers are adversely affecting the health of everyone around them against the will of those in the smokers' vicinity. No attention is paid to the smokers' health.
Depends on the particular anti-smoking campaign - seen a 'Truth' commercial lately? They seem to be concerned about more than just the second-hand smoke. I'm pretty sure if they had their way they'd ban all tobacco period.
Sometimes it is - if a person gets heart problems due to eating too much junk food, it might result in them having to get the government to front some of their health costs. This does affect other people as they are paying the taxes that pay for this, and some health advocates do note this in their campaigns. Or what if the person dies of a heart attack, who will take care of his kids? Just examples, but such logic is used by some people.
Still, regardless of the logic here the primary objection we have to these types of bans remains the same - these types of bans are people shoving their wills down the throats of others when they have no business doing so.