most places have a gate at the exit that only lets you leave by scamming your receipt
That would be unlawful detention here. Also, what about people that go in and decide they don't actually want to buy anything after all?
Fun fact: You can ignore the receipt checkers at wal-mart in the states. They have no legal authority to require you to stop. Costco, on the other hand, since it's a membership club, can.