I'm arguing in favor of legalization, not against it. In my estimation legalization means more addicts, a greater burden of addiction for society, fewer people in jail, and fewer bystanders hurt by the business. On balance it's the more moral choice, because for the most part those who end up experiencing the negative effects are doing so by choice, and dealing with legal addicts is preferable to dealing with illegal enterprises.

tl;dr Legalization makes sense, because even though more people end up negatively impacted, they are generally only hurting themselves.