It seems the most relevant terms of Williams's NPR contract have been made public, and I'd say it's pretty clear he violated certain of those terms in this case. What I don't know is how many other times he made comparable statements of personal opinion, in other venues, which he would not have been allowed to make in his position at NPR.

I do agree with Ron and others, though, that NPR handled Williams's firing in a clumsy and self-sabotaging way.