Why is WOP a derrogatory term? WOP simply stood for with out papers. It applied to every immigrant that came to the US with out papers. Who decided to make that into a negative context?
There are plenty of rednecks that run around town with "Redneck" stickers on their trucks...it obviously doesn't bug them a whole hell of a lot.
There's a big leap from calling someone a redneck to using a racial term. Redneck is a term used in everyday conversation without the threat of being slapped with discrimination charges. Use "Jap" around the workplace a few times and see how long you remain employed.