The arrest rates indicate that the law-enforcement system "believes" that black Americans use illegal drugs more — a statistical trend which isn't there.
In fact your interpretation is wrong. It is not "the law-enforcement system "believes"" that blacks use more. It is that blacks are more often dealers, and it is easier to get a conviction or plea bargain as a user than as a dealer, since the latter requires intent as well as possession and will be fought harder because of higher penalties.
I suspect that blacks are not over-represented as drug dealers. Rather, blacks live in urban areas, which can be policed at lower cost than rural areas for population density reasons.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
(I plan to make these threads from now on. Downvote if you disapprove. If I miss one, feel free to do it yourself.)