Amazon is right here. If such a review were allowed, the first people to exploit that would be the the scummy vendors themselves. They would post fake one-star reviews against competing products, claiming that all the good reviews of those products are from bribes. Always consider the secondary effects, like back-scatter from replying to a spam e-mail to give them a piece of your mind!
First round, Amazon's listings drop by 95 percent as bad actors go hog wild blocking competitors.
Second round, Amazon is forced to triage item restorations in order to put best sellers back up first.
Third round, the bad flag bans help flush out the jackasses, and minimize the incentive to come back and try again.
It'll never happen, but it's possible it could restore Amazon's reputation re caring about delivering good quality and a clean platform.