Yea, it sounds to me that the artist got paid outright and essentially sold off his rights to the artwork back when he did them. Unless he has some contract that says otherwise, he's pretty well screwed out of any royalties. It sucks and might not seem fair, but that's business. Live and learn.
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I used to be with it, but then they changed what "it" was. Now what I'm with isn't it, and what's it seems weird and scary to me, and it'll happen to you, too.