You pretty much already said it yourself... it's already a problem, it affects not just the individual but his environment, too, and generates costs unto society, too.
Now, there are cost-effective ways to solve these issues and they happen to be humane, too: support people with issues they have immense difficulty overcoming themselves, include them in society, do not stigmatise, create support structures. It's effective and carries a short-term cost.
The alternative solution to the problem is not cost-effective, not humane and in fact, not very effective: criminalise a person's problem, push him away into the worst corner of society (prison), stigmatise him (prison), fine him, take away his opportunity, and expect him to resist a drug that has a hold on his brain every waking hour without help. It doesn't work and carries a long-term cost.
And that's just the individual, let's not get into the side-effects of criminalising drugs. (I welcome anyone to look into the Mexican drug war which has taken the lives of more than 100 thousand people in the past decade, and is at least partially rooted in drug policy of various American countries including the US)
Further, I think it's important to stress the limitations of self-control. For example, we don't expect an orphan to take care of himself. We argue, because it's a kid with no level of responsibility or maturity. The libertarian case of self-responsibility does not apply here, based on the biological limitations of a kid's brain. But it's important to consider that people under the effect of drugs or addictions are also to a large extent, not a similar extent but a substantial one, emotionally/biologically limited to take adequate self-responsibility. The solution then isn't to say 'you're an adult, so get over it and take responsibility', when your brain simply has a hard time, not by choice, to act in a way that you know is rational. And in this way an addiction is similar to many other forms of disease. Similarly a depressed person can not 'just' take self-responsibility and get over it. What remains then is the argument 'but before you got addicted, you should've known better'. And that is the worst argument. For one because people don't get addicted by some clear choice, some are ignorant, some are exposed to drugs as very young children, some use it after experiencing extreme trauma. And even if you should've known better, I don't believe the solution is to withhold support as a form of punishment for the sake of punishing a bad action, that makes no sense to me.