1. More short-term crime prevention than a system catching 10% of A's crimes and 10% of B's crimes (good!)
2. Enforce a societal belief that A is intrinsically better than B (bad!)
3. Disproportionately burden children, families, and communities in B than A, causing them to indeed perform worse in everything than those in A (bad!)
4. As a result of 2 & 3 it is not a stretch to say simply causing B to do more actual crime (potentially negating point 1 entirely)
If you believe that crime enforcement is not for the sake of vengeance but instead something done to improve the well-being, safety, and happiness of citizens, you may see that inequality=bad just as crime=bad. How to best solve this trolley problem is complicated but it's important that people are aware that it is complicated before firing off an answer.
I would argue we as society don't want crime to stop simply for the sake of crime stopping (or for prison labor), but ultimately because we want to feel happy and safe from harm and unjust treatment. The systems we design need to factor in the humanness of the police and their communities and make sure they are not set up in a way that loses sight of that bigger picture.
Selective enforcement has been used historically to persecute certain minorities and that's not acceptable in a functioning society.