The reason white people dont' get offended by that is because it doesn't really mean anything. Around where I live (oh god, I'm going into this mode, oh god stop me), when I hear the word "nigger", it's rarely meant in a very friendly way. That word has a goddamn history. I don't like people using it to describe themselves... but them using it doesn't mean you can use it freely. Some black people calling themselves "niggers" doesn't mean that word has lost its connotations-- language is dependent on context. Not to mention, the person someone might call a nigger isn't necessarily the person calling himself that. The thing is, for good or ill, that word has far too many connotations to deep issues in this country.
No one is saying they have the right to not be offended. What they're saying is, that word has a history, it's a word that makes people upset, and a word that would be better off dead.
The whole "race" concept is too complicated. A very large number of anthropologists consider it to be largely a cultural distinction, it has no significant genetic basis, etc. I think the real reason it's fallen out of favor is because of its association with white supremacy, eugenics, etc.
Wikipedia says:
"In the face of these issues, some evolutionary scientists have simply abandoned the concept of race in favor of "population." What distinguishes population from previous groupings of humans by race is that it refers to a breeding population (essential to genetic calculations) and not to a biological taxon. Other evolutionary scientists have abandoned the concept of race in favor of cline (meaning, how the frequency of a trait changes along a geographic gradient). The concepts of population and cline are not, however, mutually exclusive and both are used by many evolutionary scientists.
In the face of this rejection of race by evolutionary scientists, many social scientists have replaced the word race with the word "ethnicity" to refer to self-identifying groups based on beliefs in shared religion, nationality, or race. Moreover, they understood these shared beliefs to mean that religion, nationality, and race itself are social constructs and have no objective basis in the supernatural or natural realm."
But, yeah. I think the reason people are offended by the whole oriental/occidental/negroid thing is because of the social implications, connections with racism, etc. Typically, people saying that some groups are scientifically different has led to saying that these groups are inferior or superior to each other. THis makes people uncomfortable.
As you said. Yes, you're white... so what? It doesn't mean anything to be white. no one is ashamed of being white. No one has ever thought they were inferior for being white, and if they do, they can get over it pretty quickly. As far as we've come on the race issue, people are still going to be sensitive about it. People are just sensitve about some things. Do they have the right to be sensitive? yes. Do they have the right to never be offended? no. Do they have the right to ask that you stop? yes. If someone called me a fatass, and I asked them to stop, they could either keep calling me a fatass, or they could stop. It is, obviously, their right to choose. But if they can choose, what purpose is served by calling me a fatass instead of, say, a man of gravity? Same thing for calling me a skinny little bitch, or a nigger, or a cracker, or whatever. People have the right to say it. That doesn't mean they should.