I recently got an email offering 50% off a seller's products if I left a good review. This is happening on all marketplaces, not just Amazon. The only way I see to truly solve the problem is only allowing verified buyers to leave a review.
I signed up with a throwaway e-mail account. They sent a step-by-step guide to go to Amazon, enter a specific search term, scroll until I found their product, purchase it, and then send them the receipt for reimbursement.
They didn't ask me to write a review. They only wanted to generate the appearance of organic search traffic to their product.