Regardless, the human generating and publishing these images is obviously responsible to ensure they are not violating any IP property. So they might get sued by Disney. I don't get why the AI companies would be effected in any way. Disney is not suing Blender if I render an image of Mickey Mouse with it.
Though I am sure that artists might find an likely ally in Disney against the "AI"'s when they tell them about their idea of making art-styles copyright-able Being able to monopolize art styles would be indeed a dream come true for those huge corporations.
So in effect, they are pitting Disney's understanding of copyright (maximally strict) against that of the AI companies (maximally loose).
Even if it's technically the responsibility of the user not to publish generated images that contain copyrighted content, I can't imagine that Disney is very happy with a situation where everyone can download Stable Diffusion and generate their own arbitrary artwork of Disney characters in a few minutes.
So that strategy might actually work. I wish them good luck and will restock my popcorn reserves just in case :)
The problem I see though is that both sides are billion dollar companies - and there is probably a lot of interest in AI tech within Disney themselves. So it might just as well happen that both sides find some kind of agreement that's beneficial for both of them and leaves the artists holding the bag.
There are noninfringing usecases for generating images containing Mickey Mouse - not least, Disney themselves produce thousands of images containing the mouse's likeness every year; but also parody usecases exist.
But even if you are just using SD to generate images, if we want to make sure to avoid treading on Disney's toes, the AI would need to know what Mickey Mouse looks like in order to avoid infringing trademark, too. You can feed it negative weights already if you want to get 'cartoon mouse' but not have it look like Mickey.
The AI draws what you tell it to draw. You get to choose whether or not to publish the result (the AI doesn't automatically share its results with the world). You have the ultimate liability and credit for any images so produced.
In the same way, making the model deliberately unable to generate Micky Mouse images would be much more far-reaching than just removing Micky imagery from the trainset.
Copyright infringement does generally require you to have been aware of the work you were copying. So for sure there's an issue with using AI to generate art where you could use the tool to generate you an image, which you think looks original, because you are unaware of a similar original work, so you could not be guilty of copyright infringement - but if the AI model was trained on a dataset that includes an original copyrighted work that is similar, obviously it seems like someone has infringed something there.
But that's not what we're talking about in the case of mickey mouse imagery, is it? You're not asking for images of 'utterly original uncopyrighted untrademarked cartoon mouse with big ears' and then unknowingly publishing a mouse picture that the evil AI copied from Disney without your knowledge.
I think this is exactly the problem that many artists have with imagine generators. Yes, we could all easily identify if a generated artwork contained popular Disney characters - but that's because it's Disney, owners of some of the most well-known IP in the world. The same isn't true for small artists: There is a real risk that a model reproduces parts of a lesser known copyrighted work and the user doesn't realise it.
I think this is what artists are protesting: Their works have been used as training data and will now be parts of countless generated images, all with no permission and no compensation.