Lots of companies refuse to let their products be sold at a lower price in order to protect their prices. There was one software company in particular that wrote into the contract that you could not resell the software to 3rd party. You could only sell it back to the software company. This was specialty software, and licenses averaged around $40k per seat.
The fact that Amazon willingly participated in this scheme is a strong indicator that they were getting a high volume of charge backs and returns that they had to eat because the devices were new in box but couldn't be activated.
For example, in Australia Apple locks the exchange rate at certain points in order to maintain a consistent price. When the rate moves against say the USD you can import phones at a cheaper price.
Never seen it be as high as 20% though.
How were these resellers selling iPhones and iPads for 20% off?