The problem is with making is available to everyone isn't bad because of health effects, but because of the way it's used. Every time someone goes into detail explaining why a part of marijuana is bad, it usually revolves around "it's good if you know what you're doing". Well, guess what: not everyone knows what their doing with drugs. You make it available to everyone, and suddenly you have people doing it the "wrong way". There's also the thing where someone tries marijuana, isn't affected, thinks he/she has been lied to about all drugs, and proceeds to ruin their own life.