Earlier quoted context omitted.
Ownership changes at the point of sale. At the time they do the check, they no longer have any legal basis for preventing the customer from leaving. Costco and Sam's Club can only get away with it because they can revoke membership for non-compliance. At Fry's, stopping someone for a search risks civil suit for kidnapping. Aside from that, it's a security theater tactic to discourage shoplifting, which also happens t…
Costco and Sam's check your receipt to make sure you're leaving with everything you paid for. They're much more likely to find you've left something at the checkout rather than find you shoplifting.
Finding something you may have forgotten at the checkout is the only possible benefit to the consumer, so they play it up as a point of customer service, to partially counteract the unreasonable queue they create for exiting the store.
If the genuine concern was forgotten items, they would do the additional QA check at the point of sale, where the item is most likely to be found, rather than at the exit doors.
Don't parrot the corporate propaganda without giving it your own analysis first.
The veracity is easily checked. Buy a small, disposable item with your regular purchase, and put it in your pocket after checkout. If the receipt checker notices the item is missing, note a positive result. Put the receipt in your pocket with the item. Next time around, take the item out of your pocket and put it in the cart, and see what happens. Make sure you project the confidence that yes, that object in the cart is yours, and was lawfully purchased during a previous visit. Beware that this sort of testing may get your membership revoked.