The initial campaign against cannabis was the brainchild of one LEO named Harry Anslinger.
While I'm sure his personal racial attitudes were not particularly enlightened, the outlawing of cannabis and opiates seems to have been a singleminded personal agenda he pursued across the globe from Egypt to Venezuela on law-and-order grounds.
Some say that since he worked for the Bureau of Prohibition and he saw that with the relegalization of alcohol he'd need more work, he decided to take up arms against cannabis to keep his job going.
From then to now ~ who do you think blacks call 'the man' what is he doing to hold them down?