Juan has been a Fox "contributor" since 1997. He joined NPR in 2000. They knew what they were getting.
He was their token conservative, hired to placate conservatives in congress, the CPB and major corporate sponsors.
Or perhaps he was Fox's token liberal. When he came to Fox, he had just completed a run at CNN where he had taken the liberal side on CNN's Crossfire. His 1988 book "Eyes on the Prize," and the subsequent PBS documentary, supported the role of the fedeal government against the states in the civil rights struggle. So in that case too, Juan Williams is a liberal. So Fox knew what they were getting.
Saying he came to NPR in 2000 ignores his long history with them, going back to the 1980s while he was a reporter with the Washington Post. He was a frequent paid commentator on NPR news shows.
There are a whole lot better commentators NPR could have hired if the goal was to "placate" Congress. So I have no reason to believe that idea. CPB and corporate sponsors support NPR regardless of politics. So the idea that the hiring an occasional news analyst would have something to do with CPB or major corporate funding is laughable. If he attracted money, he would have remained as a program host, and not demoted to analyst.
I wouldn't say he's either a liberal or a conservative. The word I'd use is opportunist. He'll take whatever side pays the best. The fact is he's making ten times as much money at Fox as he did at NPR. You don't get that kind of money being some bland news analyst. You need to stir things up, and he wasn't going to do that as long as he was at NPR. My sense is that this was a pre-meditated firestorm, caused by an O'Reilly producer, who suggested they could get a lot of attention of Williams said something controversial. Mission accomplished.