I should note that in the case of the comic, the text would still be fully copyrighted because it was written in a conventional way. So someone couldn't simply republish the comic, the comic as a whole would still be copyrighted. And also as I recall that wasn't a thing decided by a court but rather just by the copyright office, which is the lowest rung on the deciding-what-the-law-actually-means ladder.
In the specific case of Thaler v. Perlmutter, Thaler was making some strange claims that were pretty obviously wrong IMO and the judge was basically forced to rule that the art was public domain because every other option was kind of nonsensical.
Basically, Thaler was arguing that the AI itself should hold the copyright to the art that it had generated, and that since he was the one who had run the AI he should be assigned the copyright as a work-for-hire (like if you employ an artist in your company to make art for you, the company is assigned the copyright). Thaler was insistent that he himself didn't "make" the art.
So the judge quite reasonably went "AIs are not legal persons like humans or companies are, and in order to hold a copyright you must be a legal person. So the AI itself cannot be the copyright holder. Thaler has explicitly stated that he himself is not the copyright holder. That means that in this case there is no copyright holder for this piece of art. No copyright holder means public domain, so this piece of art is in the public domain."
The common argument for people who aren't just trying to make some kind of strange point about AI personhood like Thaler apparently was is that the AI is a tool that a human is using to make art, like a paintbrush, and so the human that used the AI is the copyright holder. As far as I'm aware this argument is far less settled because it actually requires some thought, as opposed to Thaler's which was pretty straightforward to come to a conclusion of "this is silly" about.
