Given that the executive branch has prosecutorial discretion and never has been shy of using it when it mattered, I do not believe the "treaties make us do this" argument. Maybe the federal government could not have full de-jure legalize for some technical reasons. But it is completely capable to restrain the zeal of prosecution and reduce both the number of cases and the priority of those to a wide degree. If only they wanted to. Instead, the government tries to squash the legalize even when it happens (look up Operation Choke Point[1]). It's not something forced by treaties, it's a voluntary act. Not that the US is in mortal fear of sanctions from Pakistan or Colombia for not fighting drugs enough, but it's not treaties that make police kill a teen because they suspect he has a small bag of drugs [2] or kill a senior citizen in his own bed because they suspect he has some drugs (he didn't) [3]. It's something that the government can and must fix, and no treaty prevents it from doing so. The legal technicalities can come later, but the switch from a mentality where the government is at war with its own citizens should happen now. It should have happened years ago, actually.

[1] http://www.washingtonpost.com/news/volokh-conspiracy/wp/2014...

[2] http://beforeitsnews.com/alternative/2014/05/teen-dead-after...

[3] http://reason.com/blog/2014/02/15/yet-another-example-of-why...