It's because of what the word represents. I am certainly not comfortable saying "the N word", but I see no harm in members of the black community saying it. They have taken ownership of it and made it not so threatening.
Imagine you are a black guy living in the south and 6 white guys come and say "Hey nigger, where ya going?" That's certainly a very threatening sutuation as opposed to a black guy in East St. Louis having 6 black guys come and say "Hey nigger, where ya going?"
It's a connotation that has many meanings and it's just not something us white folk should be using. How the folks in the black community use it is up to them.
So if 6 white guys approached him and said 'where do you think you are going?' , would that be threatening?
Perhaps it would be perceived as such.
So which one of those words:
should be off limits to whites, so that we don't appear threatening?
I don't use the N word. Wouldn't do it. I think it's insulting no matter the context.
It's like calling someone a m*****f***** as a joke. It's not funny, no matter who uses it or how.