Many reviewers choose not to write negative reviews out of respect for authors. Often they wish to acknowledge the effort it takes to write a book by focusing only on the positive. Additionally, they may feel that that writing a negative review is mean. A negative review, however, does not have to written in a mean spirit, nor does it mean that the effort of an author is overlooked. Rather, a negative review is the highest form of respect because it engages with the text honestly and assumes that the author, if reading any reviews, does so because they are serious enough about their craft to want to improve.
Most reviewers who offer negative critiques probably do so because they are writing for readers–not authors. By the time the book is published, the author is not going to be making any changes (usually), so critiques of how the plot was slow, the premise nonsensical, or the characterization poor are not meant to be taken as suggested areas of revision. Rather, they are meant to help readers make an informed decision about how to spend their money or their time. In this way, negative reviews show respect for readers.
However, some authors do read reviews (even if they are generally advised not to) and, in this case, a negative review is a sign of respect to them. A negative review is not a mean review–it does not attack the author personally, but rather points out areas in the book that could be improved. Serious writers want to know this. They understand that receiving only positive reviews, while gratifying, does little to help them continue to hone their craft. Negative reviews, however, provide opportunities for growth. Negative reviews, in this sense, are not negative at all.
A negative review is not a personal indictment. One negative review, or even a series of them, does not mean that a person is a terrible writer and will always remain so. Rather, a negative review is a gesture towards a possible future, one that can be created with time and effort. Criticism is not inherently bad or hurtful. Given with charity, criticism is a helping hand.