The challenging part is that these artists are protesting the use of 'style' in AI synthesized media. That is, an artist's style is being targeted (or, even, multiple artist's styles are combined in a prompt to create a new AI-original work). This is not protected by copyright—if you draw a new scene in another artist's style, it would be perhaps unethical, but legally derivative work.
If the artists who are challenging these AI systems do get there way, and they are able to legally copy-protect their "style" (like a certain way of brush strokes), this would inevitably backfire against them. To give an example: any artist whose work now too closely resembles the "style" of Studio Ghibli might be liable to copyright infringement, where before the work would be clearly derivative, or just influenced by another work, as is the case with most art over time.