Google have avoided providing windows phone users with a youtube app, instead suggested they use m.youtube.com on their mobile browser to access the videos (which display no ads - this way).
So Microsoft decided to create an app, and they did, only problem is that Google sent MS a cease and desist letter, asking them to remove and deactivate the app! The reasons mentioned:
The MS. youtube windows phone app:
(1) allows users to download videos from YouTube;
(2) prevents the display of advertisements in YouTube video playbacks;
(3) plays videos that were restricted from playback on certain platforms (e.g., mobile devices with limited feature sets).
Online remarks on this situation:
the cease and desist letter: