That's always been the reason why marijuana was illegal.
From Britannica.com:
"In the 1930s Harry J. Anslinger, head of the Federal Bureau of Narcotics, turned the battle against marijuana into an all-out war. Some believe that he was motivated less by safety concerns—the vast majority of scientists he surveyed claimed that the drug was not dangerous—and more by a desire to promote his newly created department. Whatever the impetus, Anslinger sought a federal ban on the drug, and to this end he initiated a high-profile campaign that relied heavily on racism. Anslinger claimed that the majority of pot smokers were minorities, including African Americans, and that marijuana had a negative effect on these “degenerate races,” such as inducing violence or causing insanity. Furthermore, he noted, “Reefer makes darkies think they’re as good as white men.” Perhaps even more worrisome to Anslinger was pot’s supposed threat to white women’s virtue. He believed that smoking pot would result in their having sex with black men."
I wish we weren't having the 1930s redux this year, I really do.
