It is a bit of a dirty trick, but the proof will be in the pudding. If putting your app on Amazon's store results in an overall increase in your revenue across all channels, it's worth having your app there. Conversely, if Amazon's sale-pricing of your app ends up over-cannibalizing sales on other channels with higher-percentage returns, the balance sheet will show it.
As I look at it, Amazon is essentially wagering that they can push at least 3.5x more volume of sales of your app (at 20% PPU to the developer) than you get from Google's Android Marketplace (which I believe returns 70% PPU). If they cannot do that, developers will be unlikely to continue publishing their apps there. Amazon will be looking to do this through sale pricing, bundling, coupons, targeted advertising, recommendations, and perhaps an Amazon Android phone.