@Sven Groot: I guess because the sole purpose of the application is searching, so there is marginally more justification there. Although I'm not overly convinced personally that a "search app" really fits logically into the whole "Modern UI" philosophy, but I guess they weren't going to miss the opportunity to push Bing harder.

The guideline page I was thinking of is http://msdn.microsoft.com/en-us/library/windows/apps/hh465233.aspx under the "Appropriate use of search" section.