Most parents disapprove of all drug use legal or not. Legalising does not mean glamorising cannabis use should still be discouraged even if it were legal. Legalisation could make cannabis socially acceptable, and so encourage use. Countries which have experimented with decriminalisation have not encountered massive increases in use. Legalisation could encourage use of cannabis, which in turn could encourage the use of more dangerous drugs such as ecstasy.
Minors can buy cannabis as easily as adults on the uncontrolled black market.
A legal, controlled market would restrict cannabis to adults. The legal penalties for selling drugs to minors need to be increased. Prohibition is extremely expensive to enforce the money could be better spent on education and treatment. Tax payers do not want their money spent on treating hopeless drug addicts. It is fundamental to our society that we consider adults to be intelligent enough to make decisions for themselves.