My reviews are not for sale. Nobody pays me for them. Sometimes someone sends me a free book (or more often a link to an ebook) in the hope that I will review it. But there's no payment.
Why are my reviews always positive? Three reasons:
1. I don't like writing negative reviews.
2. Panning a short story is silly; wait five minutes and it will be gone anyway.
3. Because of reasons 1. and 2. I choose to review the best story I read that week. If I didn't like any, I choose a classic.
All you other reviewers out there, if you don't get paid (and I assume you don't) maybe it's time to say so.