So? People go to jail for drunk driving and that doesn't stop people from doing it.
Yeah but how many people actually go to jail for drunk driving? Is it really worth the time and money to incarcerate someone for something as petty as smoking pot?
And actually, I've heard people propose that there should be no drunk driving law. Instead, if some drunk guy is driving like an idiot, it should be classified under reckless driving. It's not necessarily what I support since I don't know enough about alcohol/drunk driving statistics to have an informed opinion, but how would you feel about that?