Obviously, they know there's really nowhere else for third party developers to go (good luck building a competing platform), so as long as they keep their prices equal they could push that tax as high as they want. Weren't antitrust laws designed to address oligopoly situations such as this?
In a similar story, Visa and Mastercard recently settled for $7.25 billion in a class-action lawsuit related to illegally fixing their processing fees:
http://www.reuters.com/article/2012/07/13/us-creditcards-settlement-interchange-idUSBRE86C16H20120713