The society including the media creates an image that black people present with more threat. We internalize these ideas passively and they root deep down in our heart. This doesn't necessarily make us a racist, we may still make friends and get along with black people. However, under certain circumstances (e.g. when we face a danger), this bias may affect our decisions in that split second and cause the officers to overreact.
In the social experiment from the episode I mentioned, subjects were given a fake gun. They were asked to shoot at those who carried a gun, and hold their fire to those who carried a phone. In the last round of the experiment, a black guy they knew and a white guy popped up together, both carring a phone. It turned out most of the participants took the shot against the black guy, who was a friend of theirs. This didn't only apply to white partcipants, but also black, Hispanic and Asian.