I'm so tired of book groupies harping on people who give bad reviews. One of their most common attack phrases is, "If you think you can do better, write your own book!" That really bugs me, for a number of reasons.
First, it's a personal attack. It usually has little to do with the book and more to do with someone feeling insulted. Just because you like a book that other people have panned doesn't necessarily mean you're stupid. It just means your tastes are different.
(Plus, if you go around challenging people to become writers, they just might take you up on it. And then you'll have to eat your words if they write something good!)
Second, many of us could do better. But not everybody wants to be a writer. Some people are content to just be readers. And that's perfectly fine. What's not perfectly fine is to imply that, as a reader, you must like every book in existence. That's simply not realistic, and it denies the fact that we are all individuals with different opinions and tastes (and aptitudes for syntax).
Third, you don't need to be a writer to know when a book is just plain bad (unfortunately, being an editor doesn't seem to help much in some instances, either). Imagine that you go to a restaurant and order a fancy meal. But when you get your food, the salad tastes moldy, the chicken is rubbery, and the dessert is burnt. You don't need to be a five-star chef in order to know that your food tastes bad. Similarly, you don't need to be an author to know when a book is garbage.
Sometimes reviews pointing out the shortcomings of a book (or a restaurant) are necessary. If we weren't allowed to give bad reviews, we'd all have to sit there, smiling, while we choked down rubbery chicken with a side of sautéed genre fiction. And what's enjoyable about that?