Do you think reviewers are obligated to put up a good review of a book, even if they don’t like it? Have we come to a point where reviewers *need* to put up disclaimers to (hopefully) save themselves from being harassed by unhappy authors who get negative reviews?
The short answer to both of these questions is no, in my opinion. While I think that it’s important for reviewers to disclose any biases they may have in favor of or against a book, including perhaps that they received the book for free, it’s absurd to expect a reviewer to say only good things just because they got a free copy of the book, nor should a reviewer feel obligated to do so.
Any writer who expects such and would go to the lengths of harassing a reviewer who gave them a negative review needs to, in no particular order: grow up; get a life; stop writing. The business of writing and publishing is harsh, and it isn’t for the thin-skinned. Successful writers learn to deal with criticism. In fact, successful writers know that criticism is essential for them to become better writers. They learn to parse the constructive criticism from the destructive criticism. They use the former to improve themselves and their writing, and they ignore the latter. If you can’t do that, you shouldn’t be writing for anyone but yourself, your significant other, and your pets. You certainly shouldn’t be sending free copies of your books to reviewers.