Drug use is a problem for family and friends of the afflicted as well. Not to mention the surrounding community who has the economic costs and social costs of drug addiction placed on it(less productivity, more poverty, more welfare, more crime). Drug abuse isn't merely an individual issue.
OK, you convinced me. Let's jail all teenage black boys from crime ridden areas. Committing a crime would harm their family and friends and community. We can't allow that to happen even if it hasn't. It could happen, and we have to stop it before it does.