Comity and fairness go well beyond law and justice. Justice is the impartial application of the law and the law is a matter of legislation. Law gave us Treblinka and apartheid.
It is accepted that good people can do bad things: there is a difference between action and disposition. And circumstances can be extenuating. Life can be harsh; it is certainly and indubitably unjust. However, that does not necessitate that it be unfair. A will to fairness is hard-wired into our brains: we know it when we see it.
Children of 3 and 4 can tell you credibly that something is unfair.
Comity and fairness constitute the essence of the social contract. Cooperation is a superior survival strategy and gifting is both selfish and altruistic: there is a social ecology.