"what's so bad about the untagged use of AI?" this is my answer. humans have limited time. that's what makes the things we choose to spend our time on special. when a human writes a story, they're choosing to use their time to tell that particular story, even though there are other things they could be doing, or other stories they could be telling.
when readers choose to engage with a story, they're investing their limited time in an unknown outcome. they might like the story, they might hate the story, it might be the greatest thing they'd ever read. but part of the social contract between readers and writers is that it is the writer's own original writing. it is their effort that they are offering in exchange for the reader's attention.
the heart of fandom is that it is a community of people who are putting in labour for no outcome other than human-to-human enjoyment. no one is earning money. no one is "getting" anything out of participating in fandom other than comradery and community.
when people use AI to generate writing and don't tag it, there is an act of deception. it's not a story written by a person, but content generated by a machine. it is taking reader's attention under a false pretext.
my threshold for what i am willing to read if it's written by a human is completely different than if i know it's AI generated. a human with a beautiful unhinged brain may bring a brilliant horniness to a trope that i'm not otherwise that fond of and make it well worth the time to read it. a human can write a story with a take i've never considered before, or a perspective so unique it changes the way i see things.
AI can't do anything new, because it is simply averaging out everything that humans have already written.
when readers choose to spend their limited time writing comments to authors, they need to know if they're commenting on someone's actual writing or something generated by AI. it is absolutely true that some people are happy to read AI generated stories. but I'll see comments on stories that are clearly AI generated talking about how great the writing is and how much they admire the author etc. and it is my guess that at least some of the people wouldn't have chosen to leave that feedback in that way if they'd known the story was AI generated.
people can and do choose to spend their time on things created by AI. but AI can create more content than humans would ever be able to engage with. which is why we need to know if something is AI generated or not.
a human's writing is something that we can't get anywhere else. part of the deception in not tagging that something is AI generated is that we all have access to the same AI apps. do i want to read the slop churned out based on your prompts? or do i want to feed in my own prompts to get slop that is more specifically tailored to my taste? (i mean, i don't. but hypothetically.)
i think there are a lot of good reasons not to use AI to create fic (environmental, writing is great, the joy of the craft etc.), but at the end of the day, that's a choice everyone is going to make for themselves.
but we also need to be allowed to choose whether or not to engage with things that are AI generated. and using AI without tagging it takes that choice away from readers. it is tricking us into giving our limited time to a machine, even if we'd never choose to do that intentionally.