I think what you're missing is that Amazon's point, that lower ebook prices are generally good for all parties, applies to all ebooks. They do use the phrase "a particular e-book" in their example, but I assumed that the real desire is for all ebooks to get a similarly proportional discount.
Of course, even if we just consider a price drop on a single ebook, it's still a valid point that this helps that ebook compete against other ebooks that don't change their prices, and there's nothing wrong with that.
Of course, even if we just consider a price drop on a single ebook, it's still a valid point that this helps that ebook compete against other ebooks that don't change their prices, and there's nothing wrong with that.