You're ignoring the externalities of drug usage. The question becomes whether the externalities of prohibition exceed those of junkie fuckups in your extended family and community. I'm pretty libertarian overall about this stuff but you have to be in denial to fail to notice that a large fraction of the population cannot handle their drugs and alcohol. Drug users frequently ruin families and neighborhoods.

Consider, for example, that both Trayvon and Michael Brown were high on marijuana when they made decisions that lead to their deaths and ensuing hugely expensive social disturbances. If you care to step out of your upper middle class bubble, it is not at all crazy to suggest that cannabis does in fact have huge negative externalities.