I think the reason that black people get offended is the context in which the word is used i.e.: "sup my nigga?" would be using the word in a noun form referring to someone as your "nigga" a.k.a. your homie, but when used in a racial/vulgar tone it can offend anyone, not just black people. I'm white (don't flame me for saying it either cuz im not using it in a racist way) and i get pissed off when someones uses that word in such a way to degrade or insult someone of the african american culture

BTW

My black friends don't care if i call them "nigga"