In general, why is it that black women seem angry and are rude in everyday transactions?

I live on the East coast, and this behavior in black women is so common. If you smile at a black woman you are likely to get a scowl in return. Also, they are extremely rude in a customer service situation. It's like they are mad that you are "bothering" them by daring to buy something from their store. I notice it is not so bad in the west, but east coast big city black women are just plain rude.