Because they're police officers. It is and always has been the mandate of US police forces to violently repress non-whites, particularly Black people. The skin color of the cop doesn't change the function. Racism is not 'a white person did something mean to a non-white person'.
And what are you saying, without white people, there would be no need for police? PoC would just get by, no crime whatsoever? That seems unlikely to me.