I think the manager’s thought process was as follows: she was afraid of these two black men and didn’t want to directly say to them they’re kicked out of the store. She approached them to hint that they need to buy something, but her indirect advice wasn’t heeded — so she called the police to avoid having to confront them without backup.
Maybe that does show her racial bias, but hey, it’s Philly, “don’t make the black people angry” is a survival tactic.