There are certain unspoken rules governing discourse. This is called decorum, or civility, and it is a social contract between people whether strangers or friends.
For example, you generally don't say "suck my dick" to your parents, professors, or bosses.
When you transgress that contract, and there's really nothing to stop you from doing so except your own self control, do not be surprised when you're vilified for it.
Why is there a social contract surrounding n*****?
Because it is a word and words are symbols and it is a symbol of oppression, which carries with it emotional power like a cross, or a swastika. Because of that power, it is used in the creation of art or the study of it to convey certain messages to audiences, in this case, it encapsulates the struggles of the African diaspora.
It's usage between blacks and by black artists signal a shared struggle due to the social position of blacks in countries over the world. However, when the word is used by white people, it's rarely evocative of its full meaning. It becomes then a kind of kitsch, an intentional transgression of the norm because some people, particularly youth, love flirting with danger.
And this trivializes and mocks the struggles the word is meant to convey in its reclaimation by the black community.
To add insult to injury, whites, as descendents of the oppressing group have the gall to lecture the oppressed group about how they should and should not feel about their own struggle, even as they perpetuate that oppression explicitly or implicitly by disrespecting the wishes of blacks, mocking them with cockamamie hypotheticals, or putting the onus on them to stop using the symbol of their shared struggle because they deeply want to say n***** with impunity.