An important aspect here is the fact that the coffee shop employee is powerless in this situation - they could lose their job if they reply with equal rudeness.

An important aspect here is the fact that the coffee shop employee is powerless in this situation - they could lose their job if they reply with equal rudeness. How we interact with those who have less power in a situation is very revealing of our character. I can't stand it when a person is only polite to people who have some kind of leverage over them.