Mostly, those sites that are banned have forced FR to ban them through copyright lawsuits.
I dunno about that. One site of which I would have liked to bring an article is www.newswithviews.com but it was verboten. I asked "why?", but never got any kind of response. I just hope I don't get banned here for even mentioning it. There is/was something going on there that cannot be discussed, I guess.
Also, I thought that articles from HuffPo is not acceptable? (I can understand that -- HuffPo is always blatantly slanted in its reporting, IMHO.)