The window titlebar doesn't make sense in every application. In this case, the title is already displayed on the tab. Showing it in two areas that are very close to each other is just redundant.
On Windows Vista, 7, and 8, explorer does not have a titlebar since the location breadcrumb trail makes more sense. On OS X, iTunes and the App Store also do not have titlebars, favoring other ways to show information that makes more sense in the context of the app.