I've done this often too. I keep having an absurd desire, though, to start with my phone and some item off the shelf in hand, and start paying via the Apple Store App while running towards the front door, timing it so the payment goes through just before i get outside, leaving them with a mere fraction-of-a-second window between "potential shoplifter" and "valued customer". Just to see what would happen. Mostly just an interesting thought experiment.
I do wonder, if they have simply calculated that the occasional unpaid item that walks off is worth it for the goodwill generated by not having a visible security person / receipt checker on the way out. If all the merchandise is RFID tagged, they could presumably essentially generate an electronic map of the inside of the store tracking all merchandise, use bluetooth locator beacons to get very precise coordinates for all iPhones that have bought something with EasyPay, subtract out the corresponding merchandise items from the map, and then subtly alert the greeters near the door (i.e. send their phone or portable terminal a special vibrate pattern) if unpaid merchandise was heading out.