Macmillan has pressured Amazon into overpricing the Kindle editions of Macmillan books. I would not pay $15.99 for what is basically an e-file, not a book. Kindle editions are useful, portable and disposable but, as Amazon well knows, e- books are a new technology, and e-books have yet to provide many of the advantages of hard-copy. That's why Amazon sells them for $9.99; buyers are willing to put up with the limitations of e-books to save half the purchase price.
Plus, I don't like corporate blackmailers. No Macmillan books for me, Kindle edition or otherwise (and Macmillan, Amazon can tell you that I buy lots of -- as in, "many, many" -- books).