What a pile of NOT yard art he stepped in.
It turns out Amazon has a policy about author's giving gift cards to people to review their books. The policy is simple. Don't do it! It's the same as paying for a review which isn't allowed. If you give someone a gift card and they buy a new purse? Well, you paid them and they spent the money (gift card). They use it to buy your book or any book? Same thing. You paid them and they spent the money. Don't yowl about it. Just don't do it.
Panzer says, "Didn't you every hear of Smashwords? You give the reviewer a coupon code. They get the book free at Smashwords. And review it at Amazon. Problem solved."
If you want to read the whole argument, tootle over to QueryTracker Blog: Authors' New Amazon Headache: