Nobody is complaining about conservatives being banned from Democratic Underground, they're complaining about conservatives being banned from twitter and facebook.
Whereas on Breitbart you are explicitly banned for dissent, on twitter, there is only correlation.
On the other hand, any publication with an angle will favor comments supporting that view simply because this is good for circulation and thereby survival. As an exception to this, I might mention WUWT, the sceptical climate site. You can find many (or at least a reasonable showing of) comments dissenting strongly from a pro-sceptic article.