As far as I know it’s even legal to study the style of one specific artist and create imitation art as long as it’s clear that this is a new artwork from a new artist and about ~50% different from any existing works.
So a prompt like “a photo of Times Square in the style of Ansel Adams” should be ~50%+ unique.
The trickiest area for these models might be prompts that recreate existing work.
Given a prompt like “a black and white photo of Yosemite Valley in the style of Ansel Adams”, if the model is too good, then it seems like copyright infringement.
But as far as training data goes, as long as it’s acquired legally, I don’t think it can (or should) be regulated.