Great question. As an avid reader well before I was ever a writer, I think 'honest reviews' should be allowed by anyone. I paid for the books I review or disclose myself as a recipient of a complimentary copy. It all comes down to common sense, appropriateness, and sadly in some cases, a good moral compass. As an author, I can connect with my readers by sharing the books I enjoy. Passionate responses from books is not a bad thing, but how it is expresses sometimes is... In the end, I think it reflects worse on the reviewer than the author when inappropriate or malicious things are said.
Well said, Liz!