The more interesting question: What about the sources for the AI to train? How are those artists paid? Do we need to pay them? Or if it used by an AI as train data, we just say: Its like a human learning?
There is nothing that suggests it should be any different than human learning legally. As long as the output is significantly unique, it shouldn’t have any copyright issues.
Does it matter if the artist themselves rejects the idea though?
People were discussing having a "no machine learning clause" back when Copilot was being heavily scrutinized. I wouldn't be surprised if some artists allow republishing but not machine learning use. Plenty of artists already have a clause that prohibits any kind of republishing, and Danbooru is known to rehost some of those artists' content anyway until the artist notices and requests that it be taken down, if ever (for a time, they even allowed paid rewards from Patreon and other subscription services to be republished).
The original dataset from Danbooru probably contained some percentage of content that would not have been there if the original artist had noticed in time.
> Does it matter if the artist themselves rejects the idea though?
Why would it, if there are no copyright issues? No one is obligated to accept a license unless they require permission to do something which copyright would restrict. Of course redistributing the original image as part of a public dataset may be problematic, but simply using it to train an AI model—essentially the equivalent of studying it while teaching yourself to draw—is arguably not among the things covered by copyright, so you don't need a license for that and any clauses in such a license would be irrelevant.
This is also basically educational in nature, even if it's a machine rather than a human being "educated", and educational use is often exempt from copyright in some degree or another to begin with. If the dataset is restricted to non-commercial research and educational use in the right jurisdictions then even redistribution may not be an issue.
