I cannot speak for the integrity of this particular reviewer or the accuracy of this review, but as a a freelance book reviewer with almost 80 published reviews to his credit I do find it hard to believe that any of us would deliberately shirk our responsibility to provide a fair & balanced review of any title. Our task is not to be a "positive shrill" for a publisher but to offer potential book buyers a fair evaluation of the title in question. Reviewers obtain review titles from either the Journal or magazine we receive the title from (vast majority) or in some cases directly from the publisher (rarely). During my career, I have received titles both ways, and I am fully aware of how much more difficult it is to provide a fair and balanced review in the latter scenario as you always run the risk of an overly critical review seriously damaging the reviewer/publisher relationship. As the reader of a review, always ask yourself "is the title being reviewed published by the same house that published the review"? Also, most reviewers are not paid for our reviews - our usual reward is a "free" copy of the title. I myself have only been paid three times for a review.
As for the ethics of reviewers, note that we do not generally seize on one "grammatical error', one wrong historical factoid, or typo - but we should mention it if it happens frequently or if they cause us to question the author's credentials.
Also sometimes we receive titles that do not fall into our area of interest. This is clearly easier to avoid when the reviewing publication asks you to review a title then when the book publisher sends you an unsolicited review title. As a semi- professional naval and aviation historian I prefer to review titles that cover the period 1890-1945, but I have reviewed other topics and eras.
Finally, bear in mind that the reviewer is merely giving his personal "evaluation" or "feel" of the title - which may very well differ from yours & mine. If one review leaves you with questions, look up another one or two and ask yourself where do they agree and where do they disagree. If we can help you to make a more informed purchase decision, then we have done our job.
Peter M freelance book reviewer & author.