What the customer paid for is a subscription where they get to read anything they want for however long they want. They don't pay per book, so there's no "left with something they don't want".
Presumably when they stop reading Book A after the 5% mark, they would move on to Book B and Amazon will then pay the author of book B. So Amazon is paying someone for the whole duration that the customer is reading from their collection.
Personally I think it's a fair arrangement. Some of the "books" are really low effort cash grab that you'd literally open, read 3 pages and drop - it'd be unfair if they got paid just as much as well written works of the same length that you finish reading through.
Presumably when they stop reading Book A after the 5% mark, they would move on to Book B and Amazon will then pay the author of book B. So Amazon is paying someone for the whole duration that the customer is reading from their collection.
Personally I think it's a fair arrangement. Some of the "books" are really low effort cash grab that you'd literally open, read 3 pages and drop - it'd be unfair if they got paid just as much as well written works of the same length that you finish reading through.