Though I'm a pretty voracious reader, I have a hard time with reviews. If I don't like a book, I kinda don't want to criticize it — it's the old, "if you can't say something nice..." problem. So, I don't write reviews on books I don't like (and rarely write, here, about books at all. We have plenty of amazing people covering that waterfront.). I often wonder how reviewers handle that distressing little dilemma, and noticed that generally, reviews I read are just of books the reviewer liked. They don't warn you away from the stinkers, they call your attention to the good stuff, right? Well, yes, but there's another motivation, for some, as well. From Jo Walton's piece, "Why Reviewers Don't Often Say 'This Sucks'":
Lots of magazines only publish positive reviews. They don't say "You must love everything." It's much more insidious. They'll send a reviewer a pile of books and say "Here's a pile of books. Write reviews of the ones that are worth it, get them to us by Friday and we'll pay you $50 (or $100, or $25...) per review." The corrolary [sic] is that they pay nothing for the ones you don't review because they're not worth it. The reviewer is then in the unenviable position of having a pile of books they have to spend time reading before Friday, knowing they'll only be paid if they produce a positive review. Lots of people can find something nice to say about anything if it means the difference between being paid and not being paid, eating and not eating.
Makes sense, unfortunately.