Although it may make you sick and disgusted seeing it, they have the right to do it. Anyways, who else would want the domain of KKK.com? I know I wouldn't make my business with the initials of KKK.
I guess if Al Qaeda can have websites, so can the Klu Klux Klan, even though I don't subscribe to their train of thought. Freedom of speech isn't always a good thing, but what can you do? I agree that it would probably be a killer parking domain.