Ooops! Accidental premature submission.   This is what I was trying to type when I hit the wrong button:

I don't think Juan Williams or anyone else on NPR is expected to stick to a "script" based on their race, ethnicity, gender, or religion, but various token "liberals" on Fox News have acknowledged that their contracts there did restrict how often they could speak, whom they could address, and what they were allowed to say. (E.g., when Alan Colmes was a "co-host" with Sean Hannity, he was not permitted to bring up questions or issues of his own by posing them directly to a guest on the show; he could only comment on something the guest or Sean Hannity had broached as a subject, and he was limited in how much input he could have in the conversation. You will rarely, if ever, hear a truly liberal or progressive argument put forth on Fox.

As for the content of what Williams said, I think Jack The Sailor said it best:
"NPR should have simply fired him for a demonstrated lack of analytical skill."

Your example of feeling uneasy about someone who looks like Tim McVeigh coming by with a fertilizer truck makes more sense than what Williams said.   Tim McVeigh's appearance did involve a sort of insignia (the "skinhead" look) and he was driving a certain kind of truck.   You didn't say you feel uneasy every time you see someone with a buzz cut or someone driving a delivery truck.
If Williams had said, "I get a little nervous when I see 4 young middle-eastern men board a plane together and they are speaking furtively to each other in Arabic" - or something like that. That scenario would naturally evoke memories of what happened on 9-11-2001.   
But to say that every one in "Muslim garb" in an airport - regardless of age, gender, nationality, demeanor, etc. - makes you feel a little nervous, well, that is to admit to, and thus in a way legitimize a really, really irrational kind of extrapolation and prejudice.   I would not expect any "news analyst" worth his or her salt to give public voice to such irrrational prejudice. Just as I wouldn't expect any decent journalist or news analyst to go on a TV show and say, "You know, I can understand why security guards in stores watch black customers more carefully than they do white people."