The claim of theft is simple: the AI companies stole intellectual property without attribution. Knowing how AIs are trained and seeing the content they produce, I'm not sure how you can dispute that.
People claiming that backpropagation "steals" your material don't understand math or copyright.
You can hate generative tools all you want -- opinions are free -- but you're fundamentally wrong about the legality or morality at play.
See Studio Ghibli's art style being ripped off, Disney suing Midjourney, etc
Regardless of whether you think IP laws should prevent LLMs from training on works under copyright, I hardly think the situation is beyond dispute. Whether copyright itself should even exist is something many dispute.