Who writes book reviews? When I have made informal enquiries I discover that most people don't. They read a book, either an e-book or a physical book, and they love it, like it, 'it's okay' it, hate it or don't even finish it. But most don't write a review. With Amazon, the reader needs an account with the company and needs to have made one purchase at least although not necessarily the subject book. This may be to increase business or it may be to ensure that the reviewer is sufficiently motivated to jump through these hoops. Of course, once you have done that you are free to write review after review. Amazon even keeps a list of the most prolific reviewers.
Although I have not read the book or series myself, I looked at the listing for 50 Shades of Grey on Amazon specifically to discover its review status. Lots of people have read it; I believe it has sold more copies, more quickly than any other book. And sure enough, 13,000+ people have decided to review it and leave their review on line. You'd think by the first thousand or so reviews everything that there was to say, would have been said. Of note is that there are almost as many one star as five star reviews but these does not seem to have affected sales in the slightest. I have had the impression that as many readers loathe it as love it. But I'll leave that analysis to someone else.
There are apparently such entities as trolls, and I don't mean the kind that live deep in the mines of a Scandinavian country. No, these trolls, for various reasons, delight in leaving negative, insulting, and destructive comments and reviews up and down the internet. I seem to recall that fairy-tale mischief makers were called pixies but perhaps that term was deemed to be 'too cute'. They do cause grief for authors and others.
Traditional publishers send out ARCs -- Advance Reading Copies -- to lists of people, no doubt jealously guarded, who write erudite and usually favourable reviews. These can be used on back covers and first pages or publisher catalogues to encourage sales. No one star review would be found there.
What does the 'typical' reader do? Will a low star count prevent them from clicking the 'buy' button? In that case there are a lot of unanimous five star books to choose from and if nothing else it is reassuring to know that, in some cases, the writer has so many friends and relatives who will step up to the plate for them. Or it may be the best book you've ever read.