An isolated incident, or a few isolated incidents hardly make something common practice. In most states agencies are required by law to accept complaints. Many agencies even have these forms on-line so they can be completed without even coming to the front counter.
Obviously, I cannot speak for every state or every agency in the US, but by and large the ONLY way an agency knows about poor employees is because people tell them. If people do not speak up, then they get the police department they deserve.