This gets asked a lot.
The answer is because you are responsible for the actions of your employees while on the job.
This is the flip side of "for the people, by the people, of the people". I know that this is not a comfortable thing but perhaps the locals will start to pay attention to who they are hiring when it starts to hurt enough.
Actually, that's not true. You are only rsponsible for the acts of the your employess that are within the lawful scope of employment. Kicking a guy in the head, who is in diabetic shock and already on the ground in cuffs is not within the scope of lawful employment. In fact, the very essence of tghe civil rights act is to hold individuals liable who violate a person's civil rights while actingunder color of law. The Civil Rights Act calls for personal liability. Unfortunately, many municipalities have voluntarily agreed to defend and indemnify bad cops who whould otherwise have been personally liable for damages and the municipalities have purchased insurance often at a very high cost to cover the claims.