That would defeat the whole object of any kind of sales (Black Friday, Boxing Day, whatever) though. People would just buy goods in the run up and then hope for a deal before something sells out, and they could return them if they aren't in the sales and another similar item is if they bag one. It would be a logistical nightmare and I'm sure Amazon probably have the data to prove it otherwise they would just do the perceived easier thing of just refunding the difference.
But when you are in the returns period, it costs them more to get you to return the item. Not only do they have to pay for return postage, but they then end up with a used item which they have to sell at a reduced price. I've had generally good luck with getting a credit for the difference