I don’t see why they (authors/copyright holders) have any right to prevent use of their product beyond purchasing. If I legally own a copy of Game of Thrones, I should be able to do whatever the crap I want with it.
And basically, I can. I can quote parts of it, I can give it to a friend to read, I can rip out a page and tape it to the wall, I can teach my kid how to read with it.
Why should I not be allowed to train my AI with it? Why do you think it’s unethical?