The US doesn't give a damn what the UN says or wants if it's not something already in our interests. It needs no justification other than the one it's already been using, which is that it's a violation of federal law.
As for why it's prohibited by law, it seems much the same as alcohol prohibition in the twenties. Basically, if it gets you high, it's bad. Like I mentioned earlier, the reaction to synthetic pot points pretty clearly to that. It was banned in most states rather quickly, well before anybody tried to establish if it was actually harmful or not. The harm is from the intoxication. Same thing with Salvia and the new phenethylamines popping up.
Counter-cultural thinking is a deadly threat to the American way of life!
I'm not sure if that's really the case or not, although it was certainly a useful tactic in the 30's. Again, it was the "getting high" part that troubled people. Convincing America that all of our white women would get stoned and start banging negroes was just a scare tactic.
Crack, on the other hand, was entirely racially motivated. Or probably more accurately, driven by the desire to fill our new for-profit prisons with less desirable citizens. America wanted convicts, and affluent coke users are better citizens than poor crackheads.