He said specifically "race based punishment when it comes to pot possession", not arrest count. Arrest and punishment are not the same.
Police tend to target poor people because they are less likely to be able to afford a lawyer and fight prosecution. I don't think they do it on purpose, it's just the path of least resistance. Going to trial is not only more work for the officer, it also opens them up to review and internal investigation, etc.
Black people are, on average, poorer than whites and being black is an obvious visual symbol. So given the option, all else being equal, they arrest the black guy.
The same thing can be applied in the schools. Poor parents are less likely to complain if their child is subject to discipline or receives a poor grade (as they are less likely to be involved in the child's life period), and so it is easier for the school official to do those to the child with poor parents than to the child whose parents are more well off (and so more likely be involved, and to complain or take action). Black children are, on average, more likely to have poor parents.
The DOJ is informing the districts that this path of least resistance is becoming more resistant.
Is this fucking rocket science or something?
Reading the comments in this thread leads me to believe that none of "conservatives" here have ever spent a single day in business management.