Regarding teachers/profs giving students lower grades over differences of opinion, it really doesn't happen that often. Anyone who has ever attend any college worth its tuition knows that debate and disagreement are rampant on campus and in classrooms. It's part of the very fabric of the institution.

In the rare cases where it does occur, it's hard to claim you've "seen it" unless it's actually happened to you. Of course, the number of students who whine "the teacher/prof just didn't like me" is a lot larger than the number who were actually discriminated against.

And, for the cases where it does happen, I say suck it up - welcome to the real world. You go to work and are going to have bosses who will fire you for worse reasons than disagreeing with you opinions.