Why should we make an exception based on religion but not on political viewpoint? That is logically inconsistent. There's nothing special about religion.

The historical answer is because Congress wanted to be sure that employers could fire Communists for being Communists.

Of course, that's not my view. I think political affiliation should probably be protected, but it needs to be very narrow. You shouldn't be able to be fired for being a Republican. But if you post "Gay people should be executed," you shouldn't be able to hide behind "I'm a Republican, that's a political view!" any more than you should be able to hide behind "I'm a Christian, that's a religious view!"