Perhaps, but I honestly buy the "catching errors" idea. They don't have to be perfect to catch enough mistakes to identify a bad cashier (scanning an item twice

Perhaps, but I honestly buy the "catching errors" idea. They don't have to be perfect to catch enough mistakes to identify a bad cashier (scanning an item twice and missing an item are both bad). I think checking at the door is not good enough to prevent theft, especially at busy times. It's just too easy to hide stuff or blame it on the cashier forgetting to scan it. There are more cashiers than checkers, so it seems the bottleneck would be the checkers if they tried to make it thorough enough to be effective. So yeah, my guess is that the OP is correct, and that interpretation has always been my assumption.