In any situation where an employee must be available at all times, regardless of the actual traffic to the store, there is a potential for downtime. Downtime is not a sign that anyone is overpaid or that the store is not using their resources wisely. As a customer, I would much rather have someone available at a variety of hours than have a store only open a few hours a day so that they can be sure they are always super busy and getting their money's worth out of their employees.
One summer I had a 2nd job in the afternoon for about 2.5 hours after the main boss wanted to leave for the day. It was an office job and consisted of answering the phone and dealing with any walk in customers that had questions. Sometimes the phone would not ring for hours. However, they still wanted me there when it did and were willing to pay for it. So, yeah, I had to do something with my downtime or I quite literally would have fallen asleep. So I read.
I think if my manager had insisted that I do nothing but wait for the phone to ring, I would have found another job out of sheer boredom.
I do see a difference in a bank however and I can't quite pinpoint why. I think if I were the manager I would offer other activities that you could do to fill time if necessary. That way you don't have to be totally bored in between customers.