I think, and correct me if I'm wrong, that free speech does entitle you to express your opinions and also speak out against your government without fear of retribution - which is the most important aspect of it IMO.
However, there are times when people protest their right to free speech as an excuse to incite hatred and violence. And that is where, I believe, the line can be drawn.
I feel that it is the responsibility of a university professor to present a fair and unbiased, balanced viewpoint for teaching as far as they are able. A classroom is not the correct forum for pushing religious/political beliefs, but should be a forum for providing ALL of the information and encouraging students to be open-minded enough to make their own decisions.
There are plenty of other places one can express what they believe, and have access to their right for free speech, as long as the intent is not to incite hatred and violence, criminal acts and so forth. That is when it ceases to be free speech and begins to be more suspect.
The best example I can think of is that it is illegal to publicly deny the Holocaust. And I think that should be punishable. You don't have to believe in it, you don't have to agree that it happened, but to get up and denounce and deny it to others to encourage them to the same belief, in a public forum, is illegal, and it should be. That is NOT exercising your right to free speech.
I feel that the same can be said of this man - there is a time and a place, but when it stops being free speech and starts being fundamentalist radicalism - whether you are Muslim, Christian, radically right- or left-wing, environmentalist, gay rights. It doesn't matter. There's a time and a place, and the classroom is not one of them. He has every right to express his views and I don't feel he should be denied that, but to express such radical Islamic views in an American university seems inappropriate for a member of staff, whilst I feel it would be quite acceptable as a viewpoint of students, for example, who are not in the same position of authority and responsibility.