If I hire someone to write a book, I definitely owe the author a just wage. But if I do not hire the author, I do not see how I owe the author remuneration for work, since I am not employing the author.
Copying a book would make you indebted to the publisher, not directly to the author, since the publisher pays the author. If the author is getting a cut of the sales, illegally copying a book rips off both.
Also, what matters is that people receive just remuneration from their work, not as much remuneration as they can get. .

I’m not sure what you’re saying…that people should be compensated only so much for their work, and no more? Are we not entitled to ask for a raise i.e. get as much as the employer will pay us for our work, whether intellectual work or not?
A friend of mine is studying art at a university, and she seems to think doing real work without getting paid for it initially is just part of building a portfolio so that people will know that you can do good work. Writing a first novel without getting paid for it to show to potential employers that you can write novels seems analogous.
Your friend is correct, in that’s the way it works. However, the work is still her property, and as long as she legally protects it, is entitled to compensation if it is sold for profit somewhere down the line. It would be immoral, for example, if the first place where she worked took her portfolio work and sold it without an agreement on compensation to her, as it was not conceived or done while in their employment.
If the artist charged enough when selling the painting to get fair remuneration for his work, then current copyright law aside, would you owe him anything further for making greeting cards with the image from the painting on them?
My primary concern in this discussion is what should copyright law say if it should exist at all, not what is acceptable under current law.
Yes, you would owe them for their intellectual work, assuming that their work was protected by intellectual property protection laws, the details of which I’m not familiar. But morally, it’s their idea (intellectual property) and if you don’t get permission or pay for it, it’s stealing.
Stealing intellectual property is no different than stealing physical property, and yes, there is a moral basis for intellectual property laws, whether or not they are written “correctly” or “fairly”.