That is sort of like asking "If having a sexual relationship is perfectly fine what is wrong with a boss dating their direct subordinate?" - the power dynamic changes things via coercion. This isn't like A/B testing two apparently nearly equally valid curriculums on classes.

It is a self-fuffilling prophecy in the case of policing - they will skew where they find crime more where they focus their efforts. And that is assuming honest mistakes instead of outright bias laundering operations.