Second hand smoke is a thing, and it is dangerous for health... so are pollutions from cars, and most production of electricity... but other things as well. Banning everything isn't a good way.

Education and health campaign are a good way to make people willing to protect themselves and others ; or at least less of a charge on others.

Those who pay attention to depollution systems on their car pollute less, and those who pay attention to where their second hand smoke goes aren't much of a danger. And when a majority makes efforts for the comon good, those who don't make efforts are seen socially wrong. A bit as someone who doesn't shower more than once a year: it is not illegal, but the way they are seen is a social cost. And you may notice fewer people go for never washing themselves than for smoking. In my opinion, because there is nothing "cool" at never washing self. Smoking give a shady vibe, a bit gangster, a bit "cool". Treat it as a pure health problem, and it will become a sickness (and it is not "cool" to be sick).