Now let me stand on the other side of the fence.
Interesting analogy, but first and foremost copyright is very involved when it comes to marketplace assets. Both from seller and buyer perspective. When I buy some item on the marketplace Epic is granting me quite a bit of rights:
4.2.1.a Purchasing a Marketplace product grants to the purchaser a non-exclusive, worldwide, and perpetual license to download, use, copy, post, modify, promote, license, sell, publicly perform, publicly display, digitally perform, distribute, or transmit the content for personal, promotional, and/or commercial purposes under the Epic Content License Agreement.
Using your analogy imagine finding a very interesting toy… mickey mouse toy. And now start earning money on it in a way you described above. And let me know how was your experience with Disney lawyers. And it’s not that far fetched too, I’ve seen all those systems generating images with watermarks and signatures. Those systems also can generate portraits that are very familiar to famous celebrities that had their public image protected. Of course this can happen also with human artists. But it is far less likely that professional artists isn’t aware of those things and goes ahead and violates some IP rights.
On the ‘not copyrightable side’ the marketplace guidelines has this:
2.2.d Use of unmodified public domain content is limited to assisting with presentation, while not being the majority of the submission,
If the images are not copyrightable, they are essentially public domain, aren’t they? So basically those assets are in violation of that rule.
It’s complex.