Because what sort of non-conservative - one perceives Williams to be some degree of liberal; he'd probably protest that he's just a reporter; in either case, he's not a conservative - agreed to be an in-house flunky at Fox? I'm sure they offered him nice money, and money is money, and I can't say with certainty that I'd have turned it down if Rupert had waved it under my nose.
But if you're any kind of liberal at all, even in the softest and most non-political possible sense, it's basically an indefensible thing to do. Fox News wants liberalism to perish from the face of the earth. Going on their air on a regular basis and lending your name and reputation to their ideological razzle-dazzle is like agreeing to be the regular kulak guest columnist at Pravda in 1929. For "balance".I disagree. That analogy is wrong. Plus Williams was better than Colmes. Colmes is/was horrible.