Does a site like amazon only allow people who purchased the product through then leave a review? How about other sites?
Amazon could very easily put a stop to this by simply requiring that a person must have purchased the product on Amazon and have a purchasing history. Add reasonable heuristics (for example, you can't leave a review for a blender before it was delivered or for a weight loss product three days after you got it) and things could start to make sense.
Amazon can definitely make reviews legitimate and useful again but they are not interested for some reason. I don't know what that is at all. Allowing non-buyers to leave reviews makes no sense.
Some review software notes when a reviewer is a verified purchaser. I look for that when reading reviews because at least they have the product in question.
Sometimes the reviewers would be honest and say they are reviewing on behalf of company X. Sometimes not.
Anyways, all of this is a roundabout way of saying that even reviews from verified purchasers may not be 100% honest.