The problem seems trivial, especially for a company of Amazon's size. All they'd need to do is have some prominent part of the product description (say the product name) be unmodifiable without review and approval by Amazon, and have reviewers that are trained to spot patterns of fraudulent behavior. Any legitimate issues caused by the review can probably be mitigated by giving sellers the ability to get pending changes pre-approved before they go live [1].
If a seller doesn't want to go through the approval, they can delete the product listing and create a new one with a new description, but they'd lose all the reviews. Seems like a fair tradeoff.
However, all that would cost a little bit of money, and would require staff. Amazon probably made the business decision that they like money, and don't mind the costs of allowing the fraud since it's mostly borne by customers.
[1] so the Amazon review for a legitimate product name change can happen while the packaging is being redesigned, and it can all go live in a coordinated fashion