On the basis that the "machine" is not a human and there is nothing that necessitates allowing neural networks to do all the things humans are permitted to do.
Of course there is nothing that necessitates letting these systems do what humans do.
What I am saying is that I am allowed as a human to ingest copyrighted material, learn from it, form mental abstractions and use those learnings to generate new art. Why then, would I be forbidden from externalising some of that process into an artificial system that operates on similar principles.
The trained model obviously does not have agency. It is a creative and computational tool, much like any other digital tool or product.
For the same reason you aren't allowed to make unlimited photocopies of a library book.
I really feel like "reading lots of books" is a better analogy. (Of course its not perfect, since these systems can scale way past the number of books a human can read).
Usage of the model can (and will) be automated. Also, unlike you, the model scales. This is a qualitative difference in the output produced. You are left only with the superficial similarity of the input process.