That said, from the information I could glean from the article, I would be very hesitant to make this case the poster child for discrimination against conservatives on college campuses. It sounds like the student, despite the correctness of his political beliefs was a rude young man; and the professor, despite the looniness of his politics, tried to behave like a grownup in the email exchange.
As a professor, I know very well that you don't have to teach too many courses before you encounter a student in need of some maturity. Ocassionally a student on the receiving end of a 'C+' who thought he deserved a 'B-' will unleash a profane and lengthy email tirade. Most will be embarassed by their behavior if you answer them politely and reasonably, but evidently this student was very persistent. It's a waste of time to clutter up the courts with this kind of email flame war, but if a student is persistently profane or threatening, discipline within the university can be appropriate.
One of the phenomena that does occur with email is that some people will say things over email that they simply would never say to another person face-to-face. No matter who was right in the original dispute, the first person in a email exchange to resort to profanity or personal invective is almost always the loser.
"The professer he's the talkin' man" -Mark Knopfler