To be fair, this might not be as nefarious as one might think. Yes, the discrepancy in how they are enforced shows prejudice against lower income demographics. However, the drugs in the lower income areas cause more crime. For whatever economic or socioeconomic reasons drugs in lower income neighborhoods directly correlates to more crime and destruction of the neighborhood. People in rich neighborhoods don't tear up their neighborhoods. Drug use in higher income neighborhoods isn't directly contributing to crime.

Of course we can go into a slew of underlying reasons for the issue. However, as an agency already struggling and failing miserably at their job a choice needs to be made. An attainable choice providing the set of circumstances and their limited ability to change them. A DEA agent arrests some high school kids in a rich neighborhood they just took out some drug dealers. Sure, a good win, dealers off the street. However, taking down that drug gang that's been in a neighborhood has much more of an impact. Crime rates go down. Neighbors feel safer. So they focus on the most bang for their buck.