The other issue is that since there is no fixed cost associated with shipping you things throughout the year, do to the the economics of it, the plan must be to your disadvantage. Amazon isn't a charity - the deal is making them money - and therefore on average the consumers are losing out.
Take the 80/yr and split it among all your purchases. Amazon is likely spending less than that on the extra shipping speed.
PS: Hypothetically they could be spending slightly more and decreasing their profit margins in exchange for higher volumes (b/c now that you spent your 80 bucks you feel guilty about shopping elsewhere). However I'm almost certain S&H fees are a lot larger than profit margins. Since the market is already very competitive they don't have much of a profit margin in the first place.