The criminal justice system treats people very differently, depending on who you are.
In my lived experience, for middle class white kids, using cocaine, heroin, LSD, etc. was just a right of passage, that for the most part, didn’t effect their future prospects for college or a career.
Meanwhile, black and brown kids caught with a dime bag ended up going to jail.
In the US, white and black people use drugs at about the same rate, but black people are convicted at much higher rates and with longer sentences: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5614457/
(Of course there's also the odd case of a racist judge, but my impression is that it's by far the exception.)
Edit: The paper you linked does raise some very interesting questions about the motivations behind various government policies, however.
Yes, income is a factor, but that still doesn't explain why blacks are 13% of the population but the prison population is nearly 40% black.
Nobody seriously disputes the clear racial bias in the criminal justice system.
IRL, I don't know a single drug user or way how to get them. I looked it up online several times in different countries. Result always the same, a good likelihood of your delivery getting intercepted and you getting fucked as a result. Everyting about this annoys me.
During the UK conservative party leadership contest earlier this year I thought it was pretty telling that all or most of the main candidates admitted to using illegal drugs at some point.
But sometimes I forget that there are people out there who are significantly more risk-averse or perhaps more generally obedient and who see the world very differently.
I guess my point is that I didn't think it was a ridiculous question.
I'd also point out that your use of "most" here might be unwarranted. There are surveys which demonstrate more than half of respondents admitting to the use of illicit drugs [1].
[1] https://www.drugabuse.gov/national-survey-drug-use-health