I think the police being historically derived from run away slave patrols and also having a history of being linked to the KKK may play a role as well.

However, in this case-- if the story is accurate it sounds like once the guy bent down and picked up whatever it was off the ground the officer had a right to be concerned for his safety as well as others. Also why the man ran for no reason is of concern as well. Why the pastor called the police is also questionable-- I mean you'd think he could maybe talk the man down a but and assess the situation before risking getting him killed by calling the negro hunter-- I mean police.