The district attorney of St Louis has made a list of visibly racist thugs, and refuses to prosecute charges based on their say-so.

This is valid. Prosecutors' duty is not convicting whoever they can; it is doing justice. Charges submitted by racist thugs are very likely to be false, so it is correct to disregard them. There is no need to wait for each of them to be caught individually in a firing offence before discounting their accusations.

I think that display of racist attitude ought to be a firing offense, for thugs — especially if the goal is to convert it into a police department whose cops are police officers — but making that so may be difficult.