This is an interesting point. Through the DARE program, and other propaganda, cannabis has been grouped in with those other harder drugs you reference above. When someone experiments with pot and find that it's not that bad, they wonder what else have I been lied to about? Maybe this other stuff is not that bad either? Pot should have never been grouped in with the other harder drugs.
And again, yes, people can develop a problem with pot. But those same people would develop a problem with something else if pot didn't exist. Unfortunately they are just wired that way.