Racist terms applied to a race that isn't Caucasian from a Caucasian or other separate race (although more so from a Caucasian) is more insulting as there's lots of degrading history behind the use of the word. However, if a Caucasian were to be called a 'honkytonk' (racist term used against us whities), they are unlikely to feel offended - the only thing that would pass through my mind if someone called me a honkytonk is "well, I guess I won't be making friends with them." I wouldn't feel insulted.
Racist terms between separate Caucasian countries why are directed at each other exist, but there isn't degrading history behind the meaning of the word, rather it's just an insulting term, and both parties are likely to use these terms against each other openly and equally, rather than when we consider other races.
As for why Blacks call each other the degrading term that is based on Negro, well I'd say CurryMonster's 'in joke' suggestion is as close as you can get.
Sig and avatar by Ozzo. Thanks.