I've seen a number of application developers promote their software with what I will call “grey area ethics.”
In essence, they will temporarily lower the price of the application (sometimes even offering a license for free) or they will give away in-app purchases. They ask for a five-star (or extremely positive) review in exchange. In essence, they are bribing people to write positive reviews.
Book publishers do the same thing. They offer their new book for free (or for the cost of postage and handling), with the request of a highly rated review on Amazon. Or, if you purchase ten copies of their book on the day of release, they offer you hundreds of dollars in services (consulting calls, seminars, etc.) as a reward. This is a attempt to push the sales ranking up on Amazon.
For me, this is a grey area of ethics. They aren't buying positive reviews outright, but they are gaming (or hacking/taking advantage of) the review system. Every time that I see a bunch of hastily written five star reviews, I suspect that the review system was gamed at some point.
Is this a horrible crime against humanity? No. Is it a great thing for publishers to do? No. It is a grey area of questionable ethics.
I hope that people will take the time to honestly review products and not let the bribe influence their review. Put some depth and substance into your review. Don't just write, “It's great!” Say why it is. And if you don't like something about it, say it. Write the review as if you paid the full price for it. Give your honest assessment. Or, at the very least, disclose that you received the product for free or at a significant discount. Ok?