I agree that people who post AI content should be banned for plagiarism, but i believe mods don't want to start banning people for it because some of the AI detection tools give false positives if you run a post through them.
Not only because there's no accurate AI detection tool. I believe that theymos doesn't want to enforce a rule yet, this might be because AI is pretty much new and could develop to something better than spammers using it to generate posts in the forum. Making a rule against it might later contradict the forum in the future or impede the exploration of the technology.
However, even without admin's intervention, forum members have started fighting AI spammers, but it's not going to be an easy war. But in the end, people can't stop using AI to generate posts in the forum. It's better we starting learning how to accommodate it as part of forum happenings just like plagerism.