How would the woman have felt if the till operator had carried on a conversation with the till operator in the next aisle whilst serving her. Bet she would not have liked that alternative situation.
I don't know, it's happened to me more times than I care to remember, but I'm sure they'd consider it very rude of me to interrupt them and butt in. I'm not bothered. In fact, I prefer it to the inane drivel some of them come up with trying to engage me in conversation. It can be quite irritating trying to feign interest in their lives.
At the end of the day the employee is there to provide what the customer wants, annoying them by imposing their own petty values, that they seldom reciprocate isn't in the job description. I'm with marzjennings, but I'd of got her to scan the entire trolley first before walking off.