"Controlled" is the key word here.
Sure - if the FDA could "control" cannibis, it would be widely used and prescribed in the medical community by now. But since pot can be grown by the average Joe for his own consumption - medical or not - there are no major pharmaceutical companies for the FDA gestapo to extort money from for the drug's "approval".
Just one of the many examples of "government compassion".
I think you may have hit upon the answer.
The "real" reason for the opposition to MM (medical marijuana) just may be:
(a)Opposition from the drug companies
(b)Inability for the FDA to extort the proper vigorish