You are exactly correct that the favicon links to all subpages. Example the FB favicon will take you to fb.com or fb.com/obama, scrolling over the favicon will show you a preview of the result page and the url before clicking.

But even linking to different subpages I think Google would prefer users not see multiple FB, Wikipedia, ect... results and that is just one rationale I offer for Google having an interest in limiting results/page (the question of the original submission).