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Artists lose first copyright battle in the fight against AI-generated images
(www.computerworld.com)
This is a most excellent place for technology news and articles.
In the US, copyright is implicit. All work is instantly protected by copyright the moment it is created. Registering with copyright office is optional/voluntary. I think the judge's comments that you are referring to was probably referring to the works where copyright protections were waived by the artists for works placed into public domain (which, on Deviant Art, covers a vast amount).
So especially poor and young artists get exploited. Why am I not surprised.