Even if LLMs can't cite their influences with current technology, that can't be a free pass to continue things this way. Of course all data brokers resist efforts along the lines of data-lineage for themselves and they want to require it from others. Besides copyright, it's common for datasets to have all kinds of other legal encumbrances like "after paying for this dataset, you can do anything you want with it, excepting JOINs with this other dataset". Lineage is expensive and difficult but not impossible. Statements like "we're not doing data-lineage and wish we didn't have to" are always more about business operations and desired profit margins than technical feasibility.
If machines achieve sentience, does this still hold? Like, we have to license material for our sentient AI to learn from? They can't just watch a movie or read a book like a normal human could without having the ability to more easily have that material influence new derived works (unlike say Eragon, which is shamelessly Star Wars/Harry Potter/LOTR with dragons).
It will be fun to trip through these questions over the next 20 years.
If we make a machine that is capable of being as creative as humans and train it to coexist in that ecosystem then it would be fine. But that is a very unlikely case, it is much easier to make a dumb bot that plagiarizes content than to make something as creative as a human.
I disagree that our own creativity doesn't work that way: nothing is very original, our current art is based on 100k years of building up from when cave man would scrawl simple art into the stone (which they copied from nature). We are built for plagiarism, and only gross plagiarism is seen as immoral. Or perhaps, we generalize over several different sources, diluting plagiarism with abstraction?
We are still in the early days of this tech, we will be having very different conversations about it even as soon as 5 years later.