Amazon has been paying most publishers for e-book content based on the publisher's list price for said content, while charging a price chosen by Amazon. If Macmillan sets the list price to match the print edition, and Amazon pays a 30% royalty, we might have a $25 list price, and Amazon pays the publisher $7.50 out of $9.99 Amazon Kindle price. That leaves $2.49 for Amazon to enjoy. And if Amazon is paying the biggest content providers a bigger royalty, maybe not even $2.49.
Macmillan now establishes a $14.99 list price, and Amazon gets to keep 30% of that as Macmillan's agent for this sale. Suddenly, Amazon goes from getting $2.49 or less to getting $4.50.
Places, everyone? Are you ready for your close-ups?? Lights, camera, action, and then a few days later "Cut!"
Maybe from a very long term perspective Amazon would still prefer to sell the e-book for $9.99 so there's a bigger gap between the $25 print edition which Amazon sells at discount for $16.75, which makes the Kindle more attractive. But all the long term perspective aside, how angry can you be when your big customer forces you to take $4.50 instead of $2.49?