In an interview on Friday, Vivian Schiller, NPR's chief executive, defended the decision to dismiss Mr. Williams and said it was not the product of political or financial pressures.
His contract was terminated, she said, because "he had several times in the past violated our news code of ethics with things that he had said on other people's air."
On one such occasion last year, Mr. Williams said on Fox that Michelle Obama has "got this 'Stokely Carmichael in a designer dress' thing going," an allusion to a leader of the black power movement of the 1960s.
In each instance, Ms. Schiller said, "We called him on it, we had a discussion, we asked him not to do it again." NPR's ethics code states that journalists "should not express views" in other outlets, like TV shows, that "they would not air in their role as an NPR journalist."
People deserve second chances, Ms. Schiller said, but "we made the decision here because, at a certain point, if someone keeps not following your guidance, you have to make a break. And that's what we did."