There was a modest amount of Internet Fussing after Apple released its iAd Gallery app. Business Insider noted that iAd Gallery appears to violate App Store guideline 2.13, "Apps that are primarily marketing materials or advertisements will be rejected," and MacStories echoed that sentiment. Inneractive noted that a third-party developer had her app rejected for doing basically the same thing that Apple's own iAd Gallery does.
Out of all these outlets, Inneractive is the only one that focused on the real point; the third-party developer clearly had the idea for an iAd gallery long before Apple released its version, and it's unfortunate her effort went to waste. There may have been more to that rejection than meets the eye, though; it's likely that some of the iAds advertisers wouldn't have been pleased with their ads showing up in a third-party "gallery" app without their consent, and as Rene Ritchie pointed out this might have been a plausible intellectual property argument against the approval.
But even if Apple's iAd Gallery would have or should have been rejected if it was a third-party app, the key point here is it's not a third-party app. Apple doesn't have to follow the same rules as third-party developers. For instance, Apple can use private APIs and frameworks as much as it wants, and its home-grown Weather and Notes apps show it has no problem whatsoever with releasing apps that have "limited functionality" (snap).