If you download one book you're a criminal. If they download millions of books, that's just business.
Depends on your jurisdiction. In Switzerland, downloading games, books, music, movies etc. for personal use is always legal even if the copy is "pirated". Work just needs to be published in any form. Dont know any other country where it works like this.
This is mostly leftover before computers were a thing (think cassettes and paper copiers).
In practice, it's a racket and OSA is a mafia that doesn't pay to anyone. Also, the fees are rather small considering the the purpose (I think it's capped at ~$5 per device), but since authors don't actually get money from it(OSA practices) , it doesn't really matter.
Anyway, downloading audiovisual media is fine, seeding is not.