Because every mother wants their teenage son/daughter to be smoking up...
My guess is that almost every desirable "role model" has a drink from time to time, maybe more. Does that mean the parents want their kids drinking?
Don't be an idiot.
Kicks I know that the differences are really marginal but they aren't in peoples minds. There are parents who don't want their parents smoking up but don't give a shit about drinking (mine).
My point is that if a role model drinks, that's part of his personal life, and it's a non-issue. If a role model smokes, he's somehow promoting it and becomes tainted.
My parents don't care if I drink as long as they are around. As long as the green herb goes, that's a no-no. I still do it anyways, they just don't know.