I personally, and my administration’s position, is that legalisation is not the answer; that, in fact, if you think about how it would end up operating, that the capacity of a large-scale drug trade to dominate certain countries if they were allowed to operate legally without any constraint could be just as corrupting if not more corrupting than the status quo.So, it seems to me that we actually have some pretty compelling empirical evidence that this is false. It is called Pfizer, which is a large drug company that operates quite legally in the United States without, it seems, causing massive corruption. To be sure, it operates not "without any constraint" but rather with many constraints imposed specifically on drug companies, such as the FDA approval regime, and with many constraints imposed on all public companies, such as Sarbanes-Oxley and with all the constraints imposed on both public and private companies. The latter constraints, and perhaps some of the others, would operate on any legal company selling, say, marijuana, as well. Certainly, corporations operating legally are not free of corrupting influence - think Solyndra subsidies or Boeing and the Export-Import bank - but I see no reason to think that marijuana companies would be more or less corrupt, or corrupting, than other sorts of companies.
We also have the broader evidence of life before prohibition, which arrived in the early 20th century. Somehow, Americans managed to go from triviality in the 1770s to greatness at 1900 without ever banning drugs. Perhaps there are lessons there?
In short, President Obama's view is just baseless and silly.