The "If you don't like it, leave" doesn't fly with Monopoly investigations.
But the "there are plenty of other choices" defence does.
And you need to check what this complaint is about: It is about not having other stores than the AppStore. So people who can complain would be companies like Amazon, Google or Microsoft who want their stores to be available to iPhone users. As Rovio, people can buy "Angry Birds" for the iPhone, actually every single iPhone user can, so nothing to complain about. As a consumer, you can buy "Angry Birds" for the iPhone, or for other devices.