Williams violated this taboo when he talked about being nervous when he saw people in traditional Muslim attire on airplane flights and so it's not surprising that NPR chose to ritually drive him out.
What I find rather less believable is the extent that people have managed to work themselves up into a near-Victorian case of the vapors over the idea that someone could be made nervous by this kind of thing. Though admittedly, some people have managed to find mildly amusing ways of channeling their outrage such as the website Pictures of Muslims Wearing Things.
Perhaps I'm an cynical sort of fellow but it strikes me that beneath all this protest is an uncomfortable truth -- those in the secular left most outraged at Juan Williams daring to say such a thing (especially as a self-identified liberal and African American serving as the "balanced" part of the "Fair & Balanced" on the hated Fox News) feel just a nervous on flights with people in traditional Muslim garb as Juan Williams himself does. They don't admit to it, and they know that (as has been pointed out many times now) none of the people who've actually attempted to disrupt flights have been wearing anything other than normal Western clothing, but they do know that people who are Muslim have attempted to hijack or blow up flights in recent memory and that wearing certain types of clothing is a way of identifying oneself culturally with Islam. Although they may know intellectually that the individual person they are seeing is highly unlikely to be a threat (terrorists may not be brilliant, but they're smart enough to try to blend in when staging an attack) seeing someone in traditional Islamic garb reminds them that there are in fact people out there in the world who wish they could blow up the plane they are on, and that makes them nervous. Depend upon it.
Frankly, these folks are selling their ideals of tolerance short when they pretend that they aren't made nervous by the dress of certain cultures, whether that's an Arabic-looking man in a dishdasha or a young black man with low slung pants and a couple gold chains. Tolerance lies not in never having an emotional reaction to seeing someone, but rather in whether one treats all people as innocent and equal until proven otherwise.