Good Sam Card/

An update on the travails of the Flying J RVPlus card. I stopped at the Flying J in Ehrenberg, Az.,this past Friday, and used the Flying J card to get gas up at the pump, and got my discount. I then went inside the store to get some food items, and presented the same Flying J RVPlus card to the clerk. She told me that the Flying J card was not a "charge card." When I told her I had just used it to get gas, she looked at me with a puzzled look. She again said it was not a charge card, whereupon I showed her the front of the card, where it said "charge card." She then replied, "Well, I guess it's just for gas, but not for food items." I did not want to argue, so I paid with another charge card. So, my question is this. If it is a charge card, why can't I use it inside the store to purchase just about anything on sale? If she is totally wrong, then it would appear that I am about the only one who runs into some sort of new experience every time I walk into a Flying J.