I think the whole idea of sin tax, especially on liquor is to make people realise that they need to be responsible. Not stop, but be in control. It hopefully will also target the rather scary trend of the crazy treen drinkers who never think of what effect these things will have on them and their lives.
As for smoking, I think most people agree that it's not a nice habbit, and that getting people to stop is a better idea. I've known some people who are pretty poor, and don't earn a lot, but the first things they do with their cash is buy cigs and alcohol. Then comes food for the family and education for the kids and so on. That to me is just wrong.