Disclaimer: I do NOT endorse, support, or follow any of the following practices. I am describing them here for information only. I am trying to describe a rather nasty practice here, and the mentality behind it.
I’m going to have to at least partially disagree with the OP, insofar as I have, in fact, heard the n-word applied to white people, as a serious term and on a consistent basis. Several, shall we say, less enlightened members of my own extended family use the word that way to decry the failings of various groups of people, of any race or ethnicity. (Decrying the failings of pretty much every group of people other than themselves was a big pastime among my extended family, and it’s one of the big reasons I really don’t talk to them very much anymore. But that is another story.)
When talking about white people, sometimes they applied the term to groups of people of certain nationalities (for example, they might say something like, “those damned Antarctican-n*****s!”). More typically, it was applied to those rural, impoverished, and uneducated individuals that might otherwise be derided as “rednecks” or “hillbillies.” (I don’t approve of those terms either, I’m just listing some other slurs here for comparative purposes.)
In any case, whatever group it was applied to, the n-word was almost universally understood to indicate that the individuals in question were of unacceptabe countenance - worthless, shiftless, low-class, undignified, slovenly, uneducated, criminal - you name it, all rolled into one word. But in the case of white people, it seemed that there was an added factor - the term was intended to indicate that the individuals in question posessed the negative qualities listed above in such abundance that were a discredit to white people, that they were “no better than n*****s.” The implication was that being African-American was pretty much the worst thing you could be. (I am NOT saying I agree with any part of the above - but that’s how the term was used.)
For anyone out there who may wonder why African-Americans consider this little word to be so bad, well, now you know. It’s not merely impolite, or merely stupid, or merely a nasty name to call somebody. It is the embodiment of the sentiment that African-Americans are, well, everything I listed above - simply put, lower and unacceptable. Whenever some public figure is accused of having said the n-word, it seems like it gets brushed aside. Even from my own family, I’ve heard plenty of lame excuses and rationalizations, not the least of which was the “white people can be n*****s too” defense, which I regard as a big, steaming load of bullshit. For someone to have used that term is a pretty damning indictment of that individual’s character, in my view, and it is not something to be taken lightly.
So getting back to the OP: Yes, white people CAN be n*****s, but that does not make it OK. Even when the term is applied to white people, its intent is still to evoke a negative image of African-Americans.
I hope I haven’t offended anyone with the above - I know I’m walking a tightrope here. But if I’ve made you angry, about the fact that there are still individuals out there who hold these views, then my job here is done.