For example if user reads one page of one book this month this author should get 7usd.
Medium works the same way except with reading time and maybe claps.
This at least is the most logical, fraudfree and fair way to do this.
That seems illegal.
If someone pays $10/mo for access to a library of tens of thousands of books but doesn't actually read any of them, then Amazon gets income without having to pay royalties, in the same way that Netflix still gets your money if you subscribe but don't watch anything. This is true even if you decided to subscribe to KU/Netflix because "Oh, I should get around to reading Harry Potter/watching Stranger Things" and then don't get around to actually reading Harry Potter or watching Stranger Things. This is very much legal.
But does Netflix pay partial royalties on films, based on percentage viewed?