At the heart of copyright law is the intent. If an artist makes something, someone can't just come along and copy it and resell it. The intent is so that artists can make a living for their innovation.
AI training on copyrighted images and then reproducing works derived from those images in order to compete with those images in the same style breaks the intent of copyright law. Equally, it does not matter if a picture is original. If you take an artist's picture and recreate it with pixel art, there have already been cases where copyright infringement settlements have been made in favor of the original artist. Despite the original picture not being used at all, just studied. Mile's David Kind Of Bloop cover art.