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.

Pretty sure the average Redditor is AI now.

I would say people on /r/amitheasshole are more biased towards the poster, i.e. nicer.

There's plenty of those I've read where I thought it sounded like the poster was the asshole and the top replies were NTA.