Well, it never occured to me, but here it is: until the iPad was announced, Kindle had been forcing publishers to abide by it's $9.99 for a hardcover book standard. Enter Apple's evil empire: Apple will allow publishers to set their own prices, lifting the floodgates on publishers pricing their books on iTunes anywhere from $12.99 to $14.99 at their own discretion. After all, more money for the publisher means more money for Apple.
So to avoid losing big publisher Macmillan to an exclusive deal with Apple, Amazon will cave and allow Macmillan to set their own price, thereby screwing Kindle owners.
More information here.