Microsoft has brought up an issue last dealt with over a year ago. Back in a March, 2011 post regarding Microsoft's concerns about Google's anti-trust investigations, the company said that Google is preventing Microsoft from accessing data that would allow a full-featured YouTube app on Windows Phone.
What they wrote at the time:
[I]n 2010 and again more recently, Google blocked Microsoft's new Windows Phones from operating properly with YouTube. Google has enabled its own Android phones to access YouTube so that users can search for video categories, find favorites, see ratings, and so forth in the rich user interfaces offered by those phones. It's done the same thing for the iPhones offered by Apple, which doesn't offer a competing search service.
Unfortunately, Google has refused to allow Microsoft's new Windows Phones to access this YouTube metadata in the same way that Android phones and iPhones do. As a result, Microsoft's YouTube "app" on Windows Phones is basically just a browser displaying YouTube's mobile Web site, without the rich functionality offered on competing phones. Microsoft is ready to release a high quality YouTube app for Windows Phone. We just need permission to access YouTube in the way that other phones already do, permission Google has refused to provide.
Microsoft has penned another article that argues that this is still the case. "We continue to be dogged by an issue we had hoped would be resolved by now: Google continues to prevent Microsoft from offering consumers a fully featured YouTube app for the Windows Phone."
Microsoft say they are ready to release a full-featured app as soon as Google gives them permission to access certain data. You can read the full article at Microsoft's website.