Or your blog, or any number of other things where the rule is, if you can’t say anything nice, say nothing at all. This is an excellent principle to follow in general social discourse.
The minute you release a professional product for sale to the general public, it is no longer the case. The reader of a book is a consumer, and like the consumer of any other product, be it hamburgers or lawnmowers, should be free to say anything about the product that they wish. That it’s good, or it’s bad, or that they wish the pickles were pink, or anything else.
Additionally, the literary tradition is one of openness and freedom of speech. Writers and readers should abhor censorship of any kind. As writers are free to write anything they wish, within very broad parameters, so do reviewers enjoy the same freedom.