Why do some people keep their jobs when they treat their customers in a belittling manner?

i went to a department store and pajamas and under wear was on sale. i stocked up. when i got to the register the cashier made a snide remark about the combination. also do you experience this? how can they keep their jobs when i see other getting fired over this? i never report it because i feel someone else may. i am curious and really do not care as long as i get the service and get out of there.