Comment by wat10000
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!"
But if it is political/religious view? I don't quite understand how we can draw a line here. In general, belonging to a religion or political movement literally means that the subject has a set of certain explicitly stated views.