That is an odd ruling. To me it seems like if you could train an AI on legally acquired material, you should also be able to do a lot of other things with legally acquired material. AFAIK, copyright law would not allow you to make a movie featuring art on a poster you bought, or text from a book you own, or music from a CD that's yours, and to me this seems like the same thing. Now, I'm of the opinion that you should be able to do those things, at least in an incidental way (just like I don't have to pay Harder & Steenbeck every time I post a photo of a miniature I painted with an airbrush they made), but the law doesn't agree with me.