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.

I don't think Reddit is a great place to determine social consensus for well adjusted people or representative of the average adult view. I never see people on Reddit have opinions of any the people I consider reasonable in real life and I don't mean politics I wouldn't know, I don't frequent political subreddits.

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.