The current blanket prohibition of both hard and soft drugs (compare ultra-addictive and dangerous heroin to relatively benign marijuana) lumps both in the same category in the minds of impressionable children. Drug dealers stand to make greater profit off hard drugs, and so will attempt to convince users to switch from soft to hard drugs. Separating the markets through legalization will prevent this. See this to compare the numbers between the Netherlands (where the hard and soft drugs markets are separated) to the United States (where they are not).