I'm not sure where which Costco you use but I shop at the one downtown which is on the busiest in North America. Besides the parking lot being a disaster zone it's overall a rather decent experience vs WalMart or Canadian Tire. They check to see if the number of items in the cart match the amount on the bill. Theft of an item from Costco will be more than if someone swiped a can of soup from a grocery store. It's also to check that someone isn't missing an item from the cart or someone didn't pinch an item from another cart. Same goes for missed items or the possibility of a theft ring. I don't mind it. People make mistakes, loss prevention is important in any business especially one like Costco where margins are so so. It's memberships that make up the bulk of total profit. They also try to keep memberships priced the same in local markets. So the exclusive will be 120$ USD, 120$ CAD, £120. Only in Japan is membership cheaper.
My ranking of Costco. UK stores are on another level, I am sure Japan has the best stores but I've not been. Our memberships are good at any Costco
A busy Costco store at this time of year locally will be bringing in 7 figures a day in sales.