This, and in stores that are 100% self-checkout, they know exactly what you are buying before you get to a register. If you throw too many red flags (which is actually hard to do unless you are trying on purpose, which is the point) then the register holds you up and summons an employee.

The stores where they are checking receipts have too many people just walking out with shit. If you see them randomly checking people, notice that those people have the typical cashier red-flags: shit under the cart, big shit stacked on little shit w/o bags, plus the employee doing the stopping didn't see them come from the registers.