The vast majority of people in prison for drug offenses are in there for dealing, not using. So the point about blacks and whites using at the same rate is a red herring.
> "This partly reflects racial differences in the drug markets in black and white communities. In poor black neighborhoods, drugs tend to be sold outdoors, in the open. In white neighborhoods, by contrast, drug transactions typically happen indoors, often between friends and acquaintances. If you sell drugs outside, you're much more likely to get caught."
Makes sense to me. I cannot recall ever being offered drugs by a white person while I was walking down the sidewalk. (While hanging out on the beach is another story...)
To add to thatswrong0's comment. The line between possession and dealing is arbitrary, and not exactly cut and dried[1]. See also sentencing disparity in crack vs powder cocaine.
And that's not a red herring at all. Correlating use with dealing is pretty logical.