Greenwald may write opinion pieces, but the above article he wrote wasn't one of them. It's the court that decided that Rachel Maddow's viewers know that she's not always factual, not him. Here's a direct quote from the verdict, which is included in Greenwald's article:
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On one hand, a viewer who watches news channels tunes in for facts and the goings-on of the world. MSNBC indeed produces news, but this point must be juxtaposed with the fact that Maddow made the allegedly defamatory statement on her own talk show news segment where she is invited and encouraged to share her opinions with her viewers. Maddow does not keep her political views a secret, and therefore, audiences could expect her to use subjective language that comports with her political opinions.
Thus, Maddow’s show is different than a typical news segment where anchors inform viewers about the daily news. The point of Maddow’s show is for her to provide the news but also to offer her opinions as to that news. Therefore, the Court finds that the medium of the alleged defamatory statement makes it more likely that a reasonable viewer would not conclude that the contested statement implies an assertion of objective fact.
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So there you have it. Personally, I find that John Oliver's comedy show tends to be more factual than hers, and I think we can all agree that no one's going to sue him for saying some blatant untruth, because most people know that comedy shows are incredibly sarcastic. But Maddow's talk show seems fairly serious, so I'm not personally persuaded that reasonable viewers would be so dismissive of what she has to say. If you consider yourself a reasonable viewer of her show, then I think my point has been made.