Wow, it's sad when educators let their ideology bleed into the classroom. However, i certainly understand how easy it is to let that happen!
As you can probably tell by now, i have a bias towards freedom and human dignity, so it's hard not to let them come through when i teach. Still, i try to be as unbiased as possible, agnostic on the surface, and encourage students to think for themselves. I'd NEVER consider dinging someone's grade for disagreeing on a principle.
Then I salute your integrity. That's probably why I could never become a teacher: not only are my pedagogical talents very limited, but I don't think I could "objectively" grade a socialist paper, lest it would include an "interview" afterwards to see if the students' arguments can face intense scrutiny