The mythology of US racism requires us all to pretend that Black people are all liars, and that cops always tell the truth. In reality, the truth is closer to the inverse.
Through most of US history, prosecutors have kept Black people off of juries. A few pesky Black people on your jury made it much harder for police lies to just be accepted.
So during jury selection, prosecutors ask potential jurors if they "believe the police," or "have ever had negative experiences with the police."