As soon as? Lulz. Good one. But that’s inaccurate. Cannabis has never been a drug exclusively or predominantly used by minorities. That claim was propaganda promoted by special interests advocating prohibition back in the 30’s. That included changing the spelling of marihuana to marijuana in order to create an association with those lazy Mexicans. It’s true that the penalties for use are more severe for black and brown people. But there’s no evidence that they’ve ever used more than white people.
Cannabis use became much more widespread in the 50’s and 60’s so even if accurate, the “as soon as” assertion would be laughable because decriminalization/legalization is a recent development. But the most severe prohibitive measures were enacted by Nixon who made it a Schedule I drug alongside heroin in 1970. That was in retaliation against protesters of the Vietnam War who were predominantly young and white. The aim was to associate pot use with the hippies and use it as a weapon against the antiwar movement. Heroin was associated with the black community and used as cover to investigate, arrest, and jail their leaders.
There is racism in our drug laws. The best example is probably the 100-to-1 disparity in the penalty for trafficking crack vs. powder cocaine where 5g of crack got you 5 years while it took 500g of powder cocaine to get the same sentence.