It is awfully tempting to start reviewing products with glowing praise in hopes that you will soon start getting free stuff in the mail.
Case in point: the Motorola RAZR that I inherited a while back. It's a solid enough phone, got the right features, and for free it's very damned good!
But charge me $100 for it and watch my review go the other way.
Even without the arguably malevolent notion of positive-reviews-for-more-swag, the mere fact of receiving something for free likely induces a more positive review.