The UN has in fact shown interest in removing cannabis from the most restrictive schedule. The CND received the recommendation from the expert committee, and the vote was put off only because Russia and Japan are being assholes about it. This was earlier this year, it's ongoing.
Meanwhile, the feds can explicitly outlaw interference with state measures. They can pull a Bolivia, quit the treaty, and rejoin with provisions that they don't want to do something in particular (for Bolivia, Coca leaf use). And there's really nothing they can actually do if the US decides they just want to flat out ignore the Single Convention, other countries are currently ignoring parts of the treaty and they're far less powerful. There's also the possibility that they could opt out by declaring it a humans rights concern.
The Single Convention is something for drug warriors to whine about, not something that would actually stop it if we had the votes.
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For the original question in the thread, US drug laws are complete and utter bullshit, and I take every opportunity to argue this point with whoever brings it up. Legalization of all drugs is the best way to minimize the harm caused to both users and 3rd parties.