MobilityDigest calls it the app which breaks all the rules. While that is a bit of hyperbole NES EMU 7 is certainly flouting quite a few. The app is a full NES emulator (which as far as we know is not allowed) which downloads ROM under copyright from the internet (also as far as we know not allowed).
Certainly it seems to flout rule 3.1 Licensed Content, Name, Logo & Trademarks
Content allowed where:
Content and application name are original or licensed.
Copyrighted content that is used with permission. Use of branded items (logos/trademarks) has been approved by the brand owners.
and also rule 2.1
2.1 Your application must be fully functional when acquired from Windows Phone Marketplace (except for additional data as permitted below). Unless you have a pre-existing billing relationship with the user, your application may not require the user to provide payment information, within the application experience, to activate, unlock, or extend usage of the application.
At least the developer is not misrepresenting the games as his own creation, but the fact that the app clearly describes itself as a emulator makes me believe either the rules have changes or some-one is asleep at the wheel at the Marketplace Certification service.
If you want to get in on a good thing while it lasts, the emulator, which has buggy sound, can be found in Marketplace here for $5.