Also - that said - you cant simply refute the link between narcotics use and crime.
Since narcotics use has been made illegal, the two are intertwined.
Otherwise, once you criminalize ordinary behavior, which marijuana and other drug use is for many, police become the enemy, people are under threat of arrest at all times, respect for the law goes down. You're already a criminal in the eyes of the law, so venturing into physical crime is easy.
Further the relative high cost of drugs, which is largely due to their criminal status and the risks involved, leads users to commit crimes to afford the inflated prices of illegal drugs. And since there is a lot of money involved in drug dealing, guns and violence get involved.
Again, cut out the violent criminal middlemen.
Moreover, once you put someone in jail for drug-related offenses, the incarceration costs run about $40K per year, oddly keeping in sync with college tuition. Not to mention police and court costs. Drug treatment centers have to be a cheaper more humane alternative.