The most salient difference is that it's impossible to tell if an LLM is plagiarizing, whereas Xeroxing something implies specific intent to copy. It makes no sense to push liability onto LLM users.
Are you following the distinction between my scenarios (1) and (2)?
In scenario (1) the LLM is plagiarizing. But that's not the scenario we're discussing. And I already said, this is where the LLM is liable. Whether a user should be too is a different question.
But scenario (2) is what I'm discussing, as I already explained, and it's very possible to tell, because you yourself submitted the copyrighted content. All you need to do is look at whether the output is too similar to the input.
If there's some scenario where you input copyrighted material and it transforms it into different material that is also copyrighted by someone else... that is a pretty unlikely edge case.