While I'm sure there are a lot of shady sellers, a lot of legitimate sellers simply scrape the top reviewers lists and send cold-call emails offering samples/discounts in exchange for reviews.

Amazon reviewers who get discount codes or free samples are required to disclose this information in their review.

When I hit the top 1000 reviewers on Amazon Canada, I started to get offers for review samples, and as my ranking improved, I started getting even more offers, with most of them coming from Chinese vendors.