We were in another state in a store I would probably never shop in again. The savings was only 50 cents as I was just getting some licorice for the ride home. If I thought I would use it again, I would have. Plus it was a quick stop as we wanted to get home.
So you were trying to make a stop so quick that you wouldn't take the minute or two for them to swipe a new card for you yet you thought that the person ahead of you should wait and let you use her car. Maybe she was in a hurry too and didn't have a minute to spare.
However, during a medical stay in Houston, we lived across the street from Kroger and I had a card. A man did ask one day at the self check and I did let him use it. So while I do not think it's rude to refuse, I guess I would do it if it came up again. Unlikely since HEB, my regular store, doesn't require cards.