You can't fire a show host for what should amount to libel/defamation?
Free speech doesn't mean you can say whatever you want and be damned the consequences.
She's a character. She's doing what the network wants, and that's getting people fired up and talking about their shows. This thread, if I recall correctly, is talking about the show. Oddly, this thread is probably something they expected and even encourage.
Secondly, free speech does (almost) mean that you can say whatever you like and be damned the consequences. It's a lot harder to sue for defamation (television would be libel) than people think. First, she has to be flat wrong and proven so. Second, she has to have had reason to know she was wrong (hard to prove). Lastly, if she's talking about major public issues (usually the case), she also has to maliciously intend to do harm to the person she's talking about.
Add all that up, and it's very very hard to sue for defamation, at least in the US.
But the real point is that a lot of conservative, stay-at-home housewives really like her show. It's sad, but the crazies in America actually do have very big fan support.