Came here to post this, as I immediately thought of IBM's ad while I was reading Amazon's bumf. I think I would trust IBM's RFID tags vastly more than Amazon's visual tracking.
I've been expecting that for years. RFID tags capable of being read without collisions were expensive back then and needed a battery. There were problems with reading many tags at once. Those problems have apparently been solved.[1]
I suspect that Amazon has RFID tags on everything in addition to the vision systems. There are existing backup vision systems for retail checkouts, such as LaneHawk. Most of the components for this already exist. Now it's here.
I saw a fully working demo of this at IBM in 2010. It was fully RFID, plus E-Ink pricing labels on shelves so that prices/offers could be updated remotely.
Crazy to think we're actually here now. And even sans-RFID.