And you'd be wrong. The idea of receipt checking is a psychological effect on the would be criminals in making them believe they will be caught. It's the idea that it would stymie would be thieves.
The receipt checker said that because that's what they were told.
The store would rather pay someone to watch for that rather than just move all that stuff back behind the registers? What good does it do to have merchandise at a place where, if the customer picks it up, he can't pay for it?
Tell me this: if the receipt-checking isn't to catch dishonest cashiers who have accomplices, then how DO they catch dishonest cashiers? Real-time surveillance of every single checkout station? It would have to be real-time, since they want the accomplice also. And they'd have to actually find the merchandise in the accomplice's bag.
It's much cheaper to let the cashiers know that item counts are being done (even if many customers decline) and that "unrung" items will probably be found. "If you get a friend of yours to pick up 5 DVDs and you only ring up 1, you will most likely get caught and sent to jail."
Of course, this can all be defeated by having many cashiers all in cahoots with 20 or so customers. All of these 20 customers choose a checkout line staffed by one of the corrupt cashiers, and they all decline the receipt check. If security watches the video of these "suspect" customers leaving the registers, they will see that they came from many different cashiers. Nobody to blame. Logistically very difficult, and most likely a little arm-twisting would get one of the conspirators to squeal on the others.