Because his position is not one in which free speech is the aim. It is a means to an end, and that end is teaching. The specifics of what he said, and in which

Because his position is not one in which free speech is the aim. It is a means to an end, and that end is teaching. The specifics of what he said, and in which context he said it, allow discussion about the quality of his teaching - which is relevant to whether or not his contract should have been renewed.