Call me ignorant, but I don't see all rights to US cannabinoids being given to Monsanto or big Pharma. That would be a continuation of the current prohibition, essentially, as no one would /could afford it, and insurance companies sure don't want to cough that sort of monthly maintenance fee.
I think Obama avoids the conversation (and has let his DEA lap dog have her way for a while) for the single obvious reason that if he doesn't appear tougher than shit on drugs, he'll be inevitably spun as a pot head and unelectable for a second term. Romney avoids it since he can't afford to openly alienate anyone. Romney has many policies he won't discuss until after the election. MMJ is just one of them.
Personally, I'd expect Obama (assuming re-election) to simply have the Fed leave it up to the states to decide what's best.