If their religious beliefs stop them from doing their job..then they are in the wrong job. It isn't rocket science. Waitrose has a different approach and has had no such complaints from its staff apparently.
It's quite possible that it wasn't that person's job in the sense you are probably loosely referring to it.
At Christmas you could be being served by a store manager, a HR person, someone off bakery, off clothing or a temp with next to no training just chucked in the deep end. Would you rather queue longer or get served by these people with occasional things going wrong?
Most likely scenario is probably a supervisor had wandered off at that point dealing with a return + 5 different customer queries simultaneously so the person on their own initiative improvised. That or something like they said it's really really busy, can you help us out with a few customers for 30 mins. What happens if someone wants to buy alcohol, I can't sell it? Oh, don't worry we'll authorise it ourselves just buzz us, thanks very much.