Apple has had a long standing policy that they can reject apps that they offer in house.
"Apps that look similar to apps bundled on the iPhone, including the App Store, iTunes Store, and iBookstore, will be rejected."
Even when this functionality is added afterwards, Apple has the ability to remove them. The developers agreed to this when they became app developers. It is a known risk of developing a product, and so far, there has been no successful legal challenge to Apple's policy.
The question isn't about policy of Apps. The question would be to examine their policy regarding ADs as this is in regards to iAds...