Apple is making the move to have more control over App Store apps that utilize "in-app" purchases. The Sony Reader app was recently declined from App Store approval due to Apple's new stance on in-app transactions.
Until now, developers were able to sell content with in-app purchases through their own payment infrastructure. Apple has now made the stance that all in-app purchases must go through the Apple payment system, which in turn would give Apple a 30% cut off the top of sales.
This marks a shift from Apple's previous claim to want a more collaborative relationship with content producers who want to manage in-app purchases through their own backend...