I do Amazon Vine reviews and we learn quickly all the things we can't say. For health products you can hardly say anything due to the legalities of appearing to make health claims. People also get their reviews removed regularly for claiming something is inauthentic. I kind of get why, because a person probably doesn't have the equipment to really determine that, and Amazon has separate channels for reporting such things. Basically reviews are just for relating your experience of a product. There are ways of communicating lack of authenticity by being more humble, as in noting that it doesn't seem like leather, or when burned it melts like plastic. I've reviewed many e.g. fake memory cards, and had no problem noting that it has less capacity than claimed, and showing some test programs' results that confirm.

Part of the issue is that they commingle inventory their warehouses receive from third-party sellers based on ISBN. So if you receive a counterfeit, it might be the fault of the seller you bought it from, or it might be Amazon's fault for mixing in counterfeit goods from some other third-party seller without doing proper quality control. Unsurprisingly they don't want reviews that draw attention to this longstanding problem.

If Amazon put out the effort to actually combat all the shady things their marketplace helps facilitate, they wouldn't make nearly as much money.

Much cheaper to just buy out the governments (https://www.opensecrets.org/orgs/amazon-com/summary?id=D0000...) that could make legal trouble for you.