This as an extension to AdWords doesn't seem like a horrible idea. For some retailers, allowing Google to complete an order might be a net improvement over their own store/experience. For the small to medium size etailers, Google most likely can offer superior experience and an "order is an order" assuming the net profit is reasonable.
Google and Advertisers have figured out that constant small changes to search Ads prevent "PPC Blindness" and every new extension gives a temporary incremental lift in regular ad clicks.