As far as I'm aware, most of the problems that drugs in general and cannabis to a degree cause are caused by the illegal nature of the substances. The fact that they have to be bought off the street entails that they are going to be impure, unregulated, and expensive. This leads to health dangers and all the problems that are generated by an illegal trade (suppliers, dealers, runners, gangs, scoring down dark lanes etc. etc.)
There is a group of ex-cops in america that actively campaign against prohibition. In their years on the force they have seen that prohibition doesn't cure the problems that it creates- So what if you take out one dealer, all you do is create a war (excuse the terminolgy) between dealers lower down that want to take his place.
Drug users generally don't cause problems (there are alway some ****heads regardless), it's the whole system they need to employ to score that causes trouble.