Why is it justifiable? Is curiosity an offense?
Usually being fed up with asked "where are you really from?" with the implication "go back there" lots and lots of times in their lives.
I’m in California and it’s somewhat unusual for someone to be native Californian.
I wasn't offended at all, of course, but funny how context changes everything... I was actually worried they'd be "disappointed" at me for not speaking their native language.
That used to happen to me in Germany, even though my German is limited to "noch ein Bier, bitte." Whereas in France (where I lived as a kid and used to have native fluency), people would immediately switch to English. I asked one Frenchman, "why?" The answer was, "You sound French but you look Australian or maybe South African." "No, I'm American." "That can't be; Americans don't speak French like that."
In the US of course it isn't even just the overblown "stranger danger" hysteria of school children speaking to people they don't know but identifying "foreigners" due to the greater ethnic diversity in the US. And people get (justifiably) offended when asked "where are you from?" especially if they were born here and just happen to be of non-European descent.