IMHO it's not about being nice. AITA threads show an interesting phenomenon of social consensus, I think the authors wanted to show that the LLMs they checked don't have that.
It seems fairly consistently miserable in any of the common high traffic subs and you have to get down to really niche communities to see what I consider reasonable behavior that matches the behavior of people I know in real life.
Even the premise of deciding who's right and who's wrong is miserable. Most problems are like those daisy-chains of padlocks you see on gates in remote areas[0]: there are multiple factors that caused the problem, and removing any factor would remove the problem too.