You are right, AI is nothing but a tool akin to a pen or a brush.
If you draw Mickey Mouse with a pencil and you publish (and sell) the drawing who is getting the blame? Is the pencil infringing the copyright? No, it's you.
Same with AI. There is nothijg wrong with using copyrighted works to train an algorithm, but if you generate an image and it contains copyrighted materials you are getting sued.
Publicly available doesn't mean you have a license to do whatever you like with the image. If I download an image and re-upload it to my own art station or sell prints of it, that is something I can physically do because the image is public, but I'm absolutely violating copyright.
That's not an unautharized copy, it's unauthorized distribution. By the same metric me seeing the image and copying it by hand is also unauthorized copy (or reproduction is you will)
If you draw Mickey Mouse with a pencil and you publish (and sell) the drawing who is getting the blame? Is the pencil infringing the copyright? No, it's you.
Same with AI. There is nothijg wrong with using copyrighted works to train an algorithm, but if you generate an image and it contains copyrighted materials you are getting sued.