I do not think that Smoking should be R rated. I teach youth swimming lessons and I am a smoker....does this mean that I am harmful to your kids? Hell, there have been times where I have been smoking outside a store and some kid recognizes me and then I talk to there parents. And I have yet to be talked to about my appearance or actions.
It is one thing to make smoking around people who don't like smoke banned, it is another to completely shelter kids from having anything to do with it. Besides, EVERYTHING kills you....yes smoking is unhealthy, but occasionally smoking is not going to kill you in a few years