From the New York Times:
With the increase in self publishing and ebooks, there has been an increase in the need for getting the word out there, and people willing to write a review, whether or not they’ve actually read the book.
Now I have wondered whether I could make a living out of writing book reviews. I do them for free at the moment, working with the publishers (via websites such as goodreads and librarything). I can go whole months without doing a review. There are months where I wish I hadn’t even started a book, never mind then having to write a review afterwards. But I would never have thought to ask for money to write spiel on a book I hadn’t read, and especially write a 5* review on a book I’d never read or thought that was rubbish.
(By coincident there’s another blog post here that not only discusses the above, but also some other issues from the “underbelly” of publishing).
ETA: It seems that Authors are not immune to upping their own reviews and down playing other writers, even when they’re relatively successful. The Telegraph have recently pointed out that crime write R. J. Ellory has faked his own reviews.