That's really the heart of it, isn't it? Forget employer for a moment - if you said something really offensive to one of your friends, and they decided to stop being your friend, you'd be upset, but should they be allowed to no longer be your friend? Of course they should. People get to choose with whom they associate, and companies (who are not people, but that's a different post) get to choose who they hire, because those people are a reflection of them. Freedom of speech isn't freedom of consequences.