I've gotten the impression over the years, and it's JUST an impression, that some white supremicist groups are try to ride on the coat-tails of the whole Celtic thing. Riverdance got really big and lots of people found their Celtic roots. Celts are white (except the ones who aren't, Celts are Christian (some are), Celts can hate and hold grudges (who can't?), Celts have some cool symbols, including crosses, they sometimes like to drink beer, and they sometimes have festivals where you can spread the word.

White supremicist groups are mostly opportunistic infections, and will only take over an organism if they grow large enough. Imagine if you had 3 or 4 of them move to your town and decide to march around in Klan attire at your local parade. They make it onto CNN, or hit the rumor mill, and folks all over start saying things like "Oh my! I didn't know Smithville was a Klan town!"