Now, it is fully understandable and justifiable for every artist to want to protect their products from those heartless AI companies just gobbling up as much training data as physically possible. Especially artists who make a lot of money drawing but need to constantly work their asses off for it!

However
With a large chunk of artists going about this by applying image filters that either make their work look like it had oil thrown on it or like severe compression artifacts, I can't help but think this is a poor choice. Again, I understand the need and motivation, but this overall reduces the quality of their product. It adds distracting noise which also increases the image file size (that stuff adds up if you are aiming to archive lots of art!).
Many would say just live with it, and sure. But this reminds of another issue in another media industry.. DRM impacting the direct function of products, most notably games, for the sake of preventing theft. Reducing the enjoyment for their end consumer in order to have protection. For users that pirate, they not only get the product for free, they get it without DRM and thus it functions better too!
I see this happening with AI as well. It still makes plenty of regular mistakes but it has already gotten much better alarmingly fast. So now, instead of someone commissioning art that will have an ugly filter on it, they can have an AI make it for free and get it without the filter.

Is it worth giving your users a worse experience so an AI potentially can't use it, risking those users being more motivated to leave or even use AI to get cleaner results?