From a post from Microsoft in 2011:
First, in 2006 Google acquired YouTube—and since then it has put in place a growing number of technical measures to restrict competing search engines from properly accessing it for their search results. Without proper access to YouTube, Bing and other search engines cannot stand with Google on an equal footing in returning search results with links to YouTube videos and that, of course, drives more users away from competitors and to Google.
Second, in 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.
http://blogs.technet.com/b/microsoft_on_the_issues/archive/2...