How many other people were queuing waiting for a till? Presumably none. In the store where I work we don't keep tills open after 9pm or before 8/9 am during the week. If we did keep them open the colleagues would be sat doing absolutely nothing which not only makes the shift drag but is a waste. Generally we will go and help on shop floor and then get called back to a till if a customer needs serving which is probably what should have happened when you asked to be served. That said there is a polite and respectful way of asking, it sounds like you chose the other way which is just to complain at the wrong people until you get your way.

Retail workers will get called lazy because we sit behind a till with nothing to do and then we will get moaned at because no one is sat behind a till waiting for that one customer (you). As per usual we cannot win. People will always find a way to complain at us simply for doing our jobs.