"I recieved a free or discounted product in return for an honest review"
My experiences with Amazon reviewing have been somewhat unusual. A review of a smart switch I wrote received enough attention that the vendor pulled the product from Amazon. At the time of writing, I'm ranked as around the 2750th best reviewer on Amazon despite having a total of 18 reviews. But the world of Amazon reviews is even stranger than that, and the past couple of weeks have given me some insight into it.
Amazon's success is fairly phenomenal. It's estimated that there's over 50 million people in the US paying $100 a year to get free shipping on Amazon purchases, and combined with Amazon's surprisingly customer friendly service there's a lot of people with a very strong preference for choosing Amazon rather than any other retailer. If you're not on Amazon, you're hurting your sales.
And if you're an established brand, this works pretty well. Some people will search for your product directly and buy it, leaving reviews. Well reviewed products appear higher up in search results, so people searching for an item type rather than a brand will still see your product appear early in the search results, in turn driving sales. Some proportion of those customers will leave reviews, which helps keep your product high up in the results. As long as your products aren't utterly dreadful, you'll probably maintain that position.
But if you're a brand nobody's ever heard of, things are more difficult. People are unlikely to search for your product directly, so you're relying on turning up in the results for more generic terms. But if you're selling a more generic kind of item (say, a Bluetooth smart bulb) then there's probably a number of other brands nobody's ever heard of selling almost identical objects. If there's no reason for anybody to choose your product then you're probably not going to get any reviews and you're not going to move up the search rankings. Even if your product is better than the competition, a small number of sales means a tiny number of reviews. By the time that number's large enough to matter, you're probably onto a new product cycle.
In summary: if nobody's ever heard of you, you need reviews but you're probably not getting any.
The old way of doing this was to send review samples to journalists, but nobody's going to run a comprehensive review of 3000 different USB cables and even if they did almost nobody would read it before making a decision on Amazon. You need Amazon reviews, but you're not getting any. The obvious solution is to send review samples to people who will leave Amazon reviews. This is where things start getting more dubious.
Amazon run a program called Vine which is intended to solve this problem. Send samples to Amazon and they'll distribute them to a subset of trusted reviewers. These reviewers write a review as normal, and Amazon tag the review with a "Vine Voice" badge which indicates to readers that the reviewer received the product for free. But participation in Vine is apparently expensive, and so there's a proliferation of sites like Snagshout or AMZ Review Trader that use a different model. There's no requirement that you be an existing trusted reviewer and the product probably isn't free. You sign up, choose a product, receive a discount code and buy it from Amazon. You then have a couple of weeks to leave a review, and if you fail to do so you'll lose access to the service. This is completely acceptable under Amazon's rules, which state "If you receive a free or discounted product in exchange for your review, you must clearly and conspicuously disclose that fact". So far, so reasonable.
In reality it's worse than that, with several opportunities to game the system. AMZ Review Trader makes it clear to sellers that they can choose reviewers based on past reviews, giving customers an incentive to leave good reviews in order to keep receiving discounted products. Some customers take full advantage of this, leaving a giant number of 5 star reviews for products they clearly haven't tested and then (presumably) reselling them. What's surprising is that this kind of cynicism works both ways. Some sellers provide two listings for the same product, the second being significantly more expensive than the first. They then offer an attractive discount for the more expensive listing in return for a review, taking it down to approximately the same price as the original item. Once the reviews are in, they can remove the first listing and drop the price of the second to the original price point.
The end result is a bunch of reviews that are nominally honest but are tied to perverse incentives. In effect, the overall star rating tells you almost nothing - you still need to actually read the reviews to gain any insight into whether the customer actually used the product. And when you do write an honest review that the seller doesn't like, they may engage in heavy handed tactics in an attempt to make the review go away.
It's hard to avoid the conclusion that Amazon's review model is broken, but it's not obvious how to fix it. When search ranking is tied to reviews, companies have a strong incentive to do whatever it takes to obtain positive reviews. What we're left with for now is having to laboriously click through a number of products to see whether their rankings come from thoughtful and detailed reviews or are just a mass of 5 star one liners.