If you actually read the treaties, while they do set firm limitations on the legal, "non-medical" or "non-scientific" sale of schedule drugs — limits that Uruguay, Colorado and Washington ignored when legalizing cannabis — they don’t otherwise obligate countries to penalize drug use. Even the 1988 convention, the harshest of the three, which instructs countries to criminalize use, still provides an out for states, allowing such laws only as they are "subject to its constitutional principles and the basic concepts of its legal system." This loophole has been used by the Dutch to argue legally for their coffee shops.
Cannabis was condemned by the 1961 Single Convention on Narcotic Drugs as a psychoactive drug with “particularly dangerous properties” and hardly any therapeutic value. Ever since, an increasing number of countries have shown discomfort with the treaty regime’s strictures through soft defections, stretching its legal flexibility to sometimes questionable limits.
International tensions over Uruguay’s decision to regulate the cannabis market reached new levels when Raymond Yans, president of the International Narcotics Control Board (INCB), accused Uruguay of negligence with regard to public health concerns, deliberately blocking dialogue attempts and having a "pirate attitude" towards the UN conventions. President Mujica reacted angrily, declaring that someone should "tell that guy to stop lying," while Milton Romani, ambassador to the Organisation of American States (OAS), said that Yans "should consider resigning because this is not how you treat sovereign states."