I don't think it means the author has a right to all similar styles. If I can legally ask somebody to paint me something in the style of a famous (living) artist, that person presumably having seen and studied their famous works for a while, why should I not be able to ask the AI to do the same thing?
(I understand there might be people who think even a human person emulating the style of another artist is morally wrong, but at least that's a consistent argument)