I guess he's been outted.
"boohoo! You told us you were on our side...Now, you go presenting this like you're a newscaster. How dare you."
Hindraker at http://www.powerlineblog.com/ has a great analysis of why the lefty bloggers excoriate any lefty in the media who "strays" - they want the media to stay left and this attack is modest proof of the theory.