The thing is, police brutality is heavily promoted on the news right now because, well, it outrages people, that's the point. It generates more ad revenue that way.
But if you hear it and see this every day, and someone says the word "police", your mind immediately thinks of "brutality". Not that it doesn't exist, but availability bias makes the issue seem much bigger than it actually is. This is why it's a bias.
And to put the danger of racist white police officers into perspective: https://i.kym-cdn.com/photos/images/original/001/163/274/fbf...
Yes, racists are a problem, but not a very big one.