It makes sense, just not the way you think. The customer is hoping to not get a receipt, so they can get money. But in order to get the money, they have to point out they didn't get a receipt and probably call in a manager. So, the manager is "hiring" the customer, in Schneier's words, by enlisting him to start screaming whenever an employee doesn't hand over the receipt. It would have made more sense to say "The customer makes sure to keep an eye on whether or not he gets a receipt, and employee theft is reduced accordingly."

Think prolonged power outage + no curtains.