Walmart near me has started using receipt checkers lately and they don’t even really do their job, it’s kind of a gimmick really. They just look at the receipt for a second Don’t even look at your cart, but they stop you every time. It’s just such a waste of my time when I’m in a hurry. I had one person even tell me that it was required by law. No it’s not! There’s no law in the USA that says they have to check your receipt.
This was in the back of my mind the entire time I was reading your comment. They may want to stop shoppers based only on when they see red-flag items in their cart, but that would leave things open for the door hosts to (consciously or not) stop shoppers based on the appearance of the shopper themselves. To mitigate that gray area, they decide to just stop everybody.