It's pretty easy to build a convincing argument for keeping certain drugs illegal around conventional morality. Primarily, around the idea that those things which cause significant harm to others are morally wrong. Certain drugs do cause harm to people other than the users. If you legislate on a basis of common morality, then you must ban such drugs. Laws shouldn't be based on morality though. If applied thoroughly, you'd wind up with an ochlocracy which heavily restricts personal freedom, so why at all?
the ironic thing is that most of our laws are to do with morality that is born out of religious thinking. yet we know now that religion is a load of gerbil shit, we should be adjusting our laws accordingly.
'I know the voices aren't real, but man, do they ever come up with some great ideas'