Work & Politics- should your company be able to fire you for your beliefs? If yes, when?

Curious about this after all that stuff about that guy at Google who was fired for his 10 page manifesto. It's a little different because he sent it out all officially at work, but I remember reading about a deputy getting fired because his boss thought he saw him at the campaign event of a mayoral candidate he, (the boss) didn't support.

What do you think? Where is the line? I'd be really really mad if someone fired me for attending the women's march or voting democrat. But on the other hand, I think companies should be able to fire people if they find out they're like, KKK members. But I'm not sure how to define the line.