[BugsJustFindMe]

The incentive structure can get seriously misaligned if you receive the product for free. You want to keep getting free products, which means that you want to make the person giving you the products happy. Amazon is giving you the products. Amazon is happy, or at least there is a common perception to that effect, if you rate things highly because then more people buy things from them. So you rate more highly so that you can keep receiving free products.

[astura]

Not only that but, IMO, you can't really objectively review a product you didn't pay for and buy out of your own free will. "That's neat" for a product you got for free can easily be "waste of money" if you had to pay for it.