Apple swings at Amazon, Netflix
Apple isn't staying static under the day-to-day rule of COO Tim Cook. The company recently began enforcing a previously ignored rule requiring all apps that offer customers the ability to purchase media from an outside site to also allow for in-app purchases. Using Apple's API.
So basically, Amazon can't sell e-books to iPhone users unless they also offer those same titles through purchase from iTunes / some other Apple-supported system. This changes everything. But it may not be the wisest move for Apple to make just now.
Back when Apple dominated the mass smartphone market, they could afford to jerk around major retailers. But now Android is the world's leading mobile platform. Apple may be antagonizing content providers at the wrong time.
From a user standpoint though, this is pretty good news. It'll always be more convenient to make a purchase from within an app than to pop open the browser and make it. On the other hand, I don't see any rule saying the version offered in-app has to cost the same as the version purchased externally. That would be one way to keep customers from going for the "easy" option.