The problem with Marijuana has nothing to do with the health factors, whether they be good or bad. It has everything to do with the extreme difficulty when it comes to regulating its use, and the fact that there can never realistically be a forced termination of the growth (Cultivation) and sale (Drug Dealing) of Cannabis, which is what the cops are primarally after. You aren't seeing anyone in prisons for posession of a few baggies. They go for the source, the people growing it and selling it.

Once you understand this, it all makes a lot more sense.