Using up precious cocaine earmarked for white children.
If you're wondering what happened to the "black people gain drug-based superpowers and use them to commit crime" chapter of your history book, then obvious spoiler alert: It wasn't really happening. What was happening was that cocaine use among black laborers was widespread. Its recreational use was tolerated, and sometimes white employers were explicitly giving it to their workers, in both cases because they believed it would make the employees work harder. We, uh ... we used to be pretty dumb when it came to drugs.
Somehow, the "let's give our workers coke" strategy backfired, as ridiculous stories began to spread. In 1914, The New York Times ran an article claiming that "most of the attacks upon white women of the South are the direct result of the 'cocaine-crazed' Negro brain" and "Negro cocaine fiends are now a known Southern menace." While "Negro Cocaine Fiends" would be a great ironic album title, there was, shockingly, no evidence of crazed black people running wild.
While widespread use of cocaine probably wasn't great for anyone's disposition, "news" reports claimed that cocaine made black men hallucinate taunts and abuse, as well as gain incredible accuracy with guns and immunity to bullet wounds which would stop or kill a sober man. Holy shit! Why wasn't cocaine being used in secret supersoldier projects? Oh, right, because it was all bullshit. But the 1914 ban was passed anyway thanks to those myths, and not out of fact-based concerns about the health risks of cocaine. (Because white people could handle their coke, goddammit!)
If you want a silver lining, the ban largely put a stop to lynchings of black men based on the "We think he's high on coke, so he probably raped someone or whatever" clause. It also, uh, fueled nasty, often lethal stereotypes about impoverished minorities and drugs for decades to come, but that's something, right?