I used to work as a cashier at a regional grocery store in the U.S. and whenever people didn't have a membership card I would either scan the store's card or scan my own. I guess you could say I would get benefits on my card, but rarely useful ones (lets face it, I wasn't buying turkeys as a 17 year old). I mainly did it because I wanted to minimize the complaining, "but that's supposed to be that price!" and get customers in and out faster and happier. I'm not saying that all cashiers that scan other cards do it for this purpose, I'm merely suggesting that it is a possibility they are doing it for customer service and sanity reasons. In any case, I'm pretty sure I made more of a profit searching for lost coins and can return deposit tickets stuck under the belt.