Their art wasn't collected for model training. The #1 artist supposedly being copied in SD1.5 is not in the training data. Artists just don't know how the model works and think you need to put in an "art" image to get an "art" image out, but of course that's not true.
> and think you need to put in an "art" image to get an "art" image out, but of course that's not
While you don't need that to generate an image, it's something SD can actual do extremely well with ControlNet, Textual Inversion, LoRA, img2img and so on.
That's an area where things are going to get interesting in the future, as you can take any image, feed it into SD and produce hundreds of AI images from it. Very easily, without much effort and within minutes. The delineating line between original work and derivative becomes extremely blurry here, as what you are copying is not "the image", but just concepts within the image, that can be a pose, camera angle, scene layout, art style or really anything. You can "copy" it with as much variation as you want, you can remix it with other images, text prompts and so on. Where does "looking at reference" stop and "doing a copyright violation" start?
The spooky part with AI art that it stops images from being singular entities, with AI you can explode every piece into millions of possible variations. AI is so fast at generating art that a future where we could generate movies in real time might not be far away. It's already fast enough to produce images and text stories faster than you can consume them. There might be a fundamental shift in art consumption ahead of us.