I think it's assumed that OK-ing any vice, not matter what it is, will have some negative effects. But the rationales for approving (and taxing) it are:
a. People are going to do it anyway. Or, as someone once said, you can't legislate morality.
b. The positive effects, and personal responsibility, are at least equal to the negative effects. For example, of a state runs a lottery (called a "tax on the poor") and uses the proceeds to provide transportation for seniors' medical care, is that justified? I dunno, but it is a reasonable argument.
