Probably due to the inequality of enforcement. If everybody agrees something should be a crime but offender group A gets a 5 year sentence and group B gets 10 years or one gets community service and the other gets jail there's some kind of problem.

Study controlling for type of crime and circumstances finds ~10% more severe punishment for crimes black to white: http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1985377

Evidence that despite approximately equal occurrences of consumption of narcotics black citizens are arrested much more frequently than white citizens: https://www.washingtonpost.com/news/wonk/wp/2013/06/04/the-b...