Most teens, or professors for that matter, do not take their moral logic so far, of course. But we should recognize this new model is not some aberration, not a form of psychopathology. It is cross-culturally a very common way for humans to approach morality, a.k.a. Those who belong to my clan, I'm obliged to treat well. Those who are outside my clan are outside my moral concern.
Morality is personal.
What's right for you isn't necessarily right for me. Kant called it the universal imperative. Belief in an absolute moral code transcending personal tastes and affections is not an inevitability. It is the product of two millennia of thinking and teaching about God, reason and human nature. And it can disappear, it seems, virtually overnight.