Well there are always some dimwits here who assume that the customer is being abusive rather than simply expecting the assistant to do the work they are paid for, rather than chatting to their equally brainless mates.
If one of my staff had spoken to a customer like that, no matter what the provocation, they would have been on report to the area boss.
From my own experience of managing staff in a customer facing environment I know that many employees today often do not have the initiative to find themselves a task to do during slack periods, but just stand around looking bored, chatting or checking their wretched phones. In the first job I ever started (age 14) I was told to keep busy by tidying up or brushing the floor etc when there were no customers. It made the time pass more quickly.