Comment Also their service is optional (Score 2) 172
You don't have to have your stuff on their subscription services. It is up to the author (or publisher, whoever controls the copyright). You can have all, some or none of your stuff on their subscription services. However, many choose to have stuff on subscription because it helps people discover your stuff, and while you may not make a lot per view/listen, you make some and it can add up.
Pay per page view actually makes sense, as it helps reward authors that release stuff worth reading. If you do pay per book, then someone can release a book that look interesting, but has no substance. However if people have started reading, well they got their money, and they are done. With page views counting, then it is the stuff that is quality that people read to the end that gets rewarded.