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.
As everyone has said, they have no legal basis to stop you from walking out, unless they’re asserting shopkeeper’s privilege, which has certain limits and opens them up to possible liability. This is why employees are trained that only managers and Loss Prevention can do that.
However, they absolutely are able to ban you from a store for not complying. How effective a ban is at a store that sees thousands of people walking in every day, I’m not sure. But it’s something to keep in mind if you have limited other options nearby.