Jo Walton gives several reasons — here are two:
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 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. I was sometimes in this position when I reviewed for the old British RPG magazine Arcane. I tried hard to be ethical and often succeeded. Only publishing positive reviews is as terrible an idea as it was when Orwell wrote against it.
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Beyond that, there’s an extra layer if you are a writer reviewing—and this is why I stopped writing about everything I read. If you are a writer, there’s a way in which all the other writers are your competition. This is quite different from them being your friends. You’re competing: for awards, for review space, for attention, for sales. I don’t think it’s a zero sum game like this so that if someone buys my book they don’t buy someone else’s. But some people do. In this worldview, if I trash someone’s current book, not only am I hurting their potential sales, but they imagine I am doing it on purpose to put down a rival. The fact that this never occurred to me before someone accused me of doing it—on my livejournal ages ago, not here—is irrelevant. That was their perception. And I can only deny intentionality. I’m a midlist writer. I’m also a reader. From my point of view, I was a reader warning other readers to avoid a bad book. From that author’s point of view, I was one midlist writer putting down another midlist writer to my own potential advantage. This is so repulsive a thing to have thought about one that I’ve been extremely careful ever since.
And, there is more in Walton’s piece; take a look.
