Cost of distribution maybe free but attracting qualified and useful reviews is definitely not.
People who think open access is the solution to that are dealing with a focusing illusion - https://www.edge.org/response-detail/11984
People who think open access is the solution to that are dealing with a focusing illusion - https://www.edge.org/response-detail/11984
Amazon reviews are not remotely a good comparison. There is huge incentive in terms of experience gain, getting a bit of a sneak peek at research, and reputation gain(lets be real, its not that uncommon for people to be fairly confident they know who an anonymous reviewer etc is).
I'm not trying to make any point other than to add further disagreement to the idea that these reviews are not currently obtained essentially for free.