Ah yes, the old "problem" of liberals in academia. This is a topic I will comment on in more detail at some point, but for now go read the Samuel Taylor Coleridge Foundation's post about being a liberal student at a very conservative college. Here is an excerpt:
I'll always especially remember the final exam in a Creation Science class (yes: we actually had one, and it was required for graduation), when the main essay question required us to write why we agreed with the conclusions in a creation science text we'd read*, and my answer was instead a refutation of them...I left the test sure I'd be failed, but I got an A+ from two very biased professors, who graded me on my argument, not on my content.
This is a lesson the David Horowitzes of the world never take the time to learn. While no doubt there are some faculty who do grade on ideology -- on the right and left -- in my experience college and university faculty are far more concerned about half-assed reasoning, poor use of evidence, uncritical parroting of sources and obstinate failure to consider alternate points of view. These are sins across the ideological spectrum -- right, left and center.
When a conservative student, egged on by gadflies like Horowitz, blames the liberalism of a professor for giving him or her a D for a poorly written paper, it's just another case of smearing the messenger in order to ignore the message. They ought to spend more time studying and writing and less time complaining, let alone filing lawsuits because they're asked to read a book.