So can cars, steak knives, firearms, chainsaws and aspirin. I don't consider alcohol or cannabis use to be immoral and even if I did I wouldn't feel justified in telling someone else what they can or can't do provided it doesn't harm anyone else.Certainly it can be argued that both alcohol and marijuana use can lead to harm to others; it demonstrably does in domestic violence and DUIs.
The prohibition on cannabis and prostitution creates many more of the problems than the substance or act itself.