Why do libs attack Fox News for having a "Right wing bias" but never mention the blatant slant on MSNBC?

On Fox, both Geraldo and Alan Colmes have their own shows. Also, O'Reilly, Hannity, and Huckabee frequently have liberal guests on their shows. When was the last time Rachel Maddow had someone on her show who disagreed with her point of view? Is she afraid to have someone such as Ann Coulter be a guest because she will look like a fool? I still have yet to see one airing of "The Last Word w/ Lawrence O'donnell" where where he hasn't said something blatantly slanderous or false.