From the amazon web site:
….ultimately, however, we will have to capitulate and accept Macmillan’s terms because Macmillan has a monopoly over their own titles, and we will want to offer them to you even at prices we believe are needlessly high for e-books. Amazon customers will at that point decide for themselves whether they believe it’s reasonable to pay $14.99 for a bestselling e-book…..
For some back ground and discussion, see this earlier post and links among the comments that describe the situation.