That's such an inane question... That's like saying "The flu only affects those that aren't immune to it, why bother giving flu shots?"

At the very least it's a public health issue. In its most academic form, drug trade has a broad number of negative externalities (organized crime, sex trade, gang violence, arms dealing, government resource drain, corruption, etc.) Your claim that drug users harm only themselves is convenient in a vacuum, but it's abhorrently ignorant in reality. By participating in a market for illicit substances, buyers are just as responsible for the negative (and positive, if there were any) externalities as sellers. The fact that punishing buyers is also more effective in shutting down the market (the supposed goal) certainly doesn't help their case for innocence.