Microsoft has been pushing strong to ensure that Windows Phone 7 has a successful marketplace. Not only has Microsoft put a lot of focus on getting developers, but they also have implemented limits to help ensure the marketplace remains high in quality. 

One of the limits Microsoft has set is to only allow "approved" developers to use Xbox Live services, such has gamer scores and achievements. An article from Ozymandias states (quote):

"Anyone can develop any title they wish using the Windows Phone Developer Tools; there’s no obligation or requirement to use Xbox LIVE or other services in those titles. However, titles wishing to use Xbox LIVE services must have a relationship with Microsoft; if you’re interested in being considered for our portfolio (and use of Xbox LIVE services), you can mail wpgames@microsoft.com. If you’ve got the right stuff, we’ll hook you up!"

This limitation is in place to help make sure that Xbox Live titles remain high-quality and that developers of said titles will receive a fair share for their work, rather than being jumbled up with several mixed-quality applications.

[Via: 1800PocketPc]