It sounds like it was an amendment introduced by a confused lawmaker who wanted to make sure the law wouldn't stop companies from firing Communists. The amendment was then accepted on the basis that it didn't make any difference (since as you note party affiliation isn't protected in the first place), so it wouldn't do any harm to include it.
http://lawdigitalcommons.bc.edu/cgi/viewcontent.cgi?article=...
It sounds like it was an amendment introduced by a confused lawmaker who wanted to make sure the law wouldn't stop companies from firing Communists. The amendment was then accepted on the basis that it didn't make any difference (since as you note party affiliation isn't protected in the first place), so it wouldn't do any harm to include it.