Also, a lot of people move from one drug to another because their current drug isn't getting them high enough anymore. I don't see how putting the weed in shops stops people from moving onto other drugs.
Of course, it doesn't stop it completely, but there have been tremendous drops in the amount of people getting into different things from marijuana use. This is because they've effectively separated the pot crowds from the harder drug crowds. People no longer have to meet a drug dealer on a dark corner in the middle of downtown at 3 am just to get a gram of weed.
The illegalization of marijuana is actually a relatively recent occurance. At the beginning of the 20th century it was legal worldwide. It eventually became illegal in the United States for a variety of reasons, the main ones being that it simply became popular among black people, and the other being the impact that hemp had on the pulp and paper industry as a viable alternative. This led the big-wigs to lobby for illegalization based on narcotic use. Around the 1950's the US began to pressure the rest of the world for illegalization as well.
Sure, it's not good for you to smoke pot, but it's nowhere near as bad for you as smoking cigarettes and drinking on a regular basis. Comparatively it's practically not addicting at all, and it doesn't destroy as many organs, or affect them as fast as smoking and drinking.
Also, a lot of people move from one drug to another because their current drug isn't getting them high enough anymore. I don't see how putting the weed in shops stops people from moving onto other drugs.