Amazon tends to remove reviews in waves. I suspect this is so that they can use known fraudulent reviewers to catch unknown fraudulent reviewers.
As a seller, it was a great day to see my number one and two competitor have their product reviews dropped to almost 0 overnight. I didn't lose a single review. I hope Amazon keeps up what they're doing because it will only make things better.
P.S. Other things to look at are:
1. Many reviews in a short time frame (Only 1-3% of people leave a product review, so if you see 20 reviews in 3 days, they would have needed to sell almost 700 units! Not impossible for a very well established product, but a new one? Almost impossible, report it.)
2. The very first reviews. These are almost always fake and unverified. No one ever looks for this but it is the most common indicator in my experience of faked reviews given how low the bar is.
After protecting yourself, protect others! The best thing you can possibly do is to report violations!
The unverified part is no longer true. A lot of companies now get reviewers to buy the product from Amazon and reimburse them for their costs via PayPal. So in general, when it comes to obscure Chinese brands, you can't trust the Verified Purchase label any more. I stopped doing incentivized reviews with the rule changes, but I know that's how they operate, as I've had many offers to go the PayPal route.
I rely too much on Amazon for my own purchases, and it wasn't worth getting banned from reviewing for that.
What you're talking about tends to happen over facebook or wechat. But there are "Amazon Launch Services" which are far more pervasive in my experience. You'll find countless services like Viral Launch, JumpSend, etc. that find some way to make it hassle free to get those first few crucial reviews.
The funniest thing I've ever seen happen is one person was scamming the sellers. They would place the order, get paid via PayPal, and then cancel the Amazon order. It was cathartically beautiful.
Possibly for new items in the mainstream.
However for the long-tail of niche items the plurality of my Amazon reviews were the first and often they remain the only, particularly for older or out-of-print books.
Books on the other hand are probably sold by sellers who bought them used/returned/wholesale and are flipping them. They could care less about your product review because they almost always have only one or two copies.
Those are legacy reviews. You're not allowed to do that anymore [source: I used to write incentived reviews]. Amazon has deleted a lot of reviews like that with their new rule changes.
The problem is not "I received this product in exchange for a fair and honest review" -- that statement is a result of Amazon's old rules, which required disclosure. This is not very different from a product getting a Vine review (I was also a Vine reviewer for a time). Vine reviews tend to skew to the positive side in spite of Amazon telling reviewers to be honest (but to be fair, most of the Vine stuff I was getting were indeed good products).
Not all reviewers who used that boilerplate disclosure statement were dishonest reviewers (I would like to think I was honest, but everyone's entitled their own perspective).
In my case - I didn't accept everything that was offered to me, just stuff that looked like it was decent quality that I thought I would like. I reviewed everything in the context of their selling price. Most of the stuff I got sold for cheap, and were surprisingly good for that price. The higher the price of the product, the more critical I was. I'm not going to review a $20 fitness band with the same critical eye as a $300 band, because in general, you do get what you pay for.
The required disclosure of "I received this product in exchange for a fair and honest review" is actually useful, because it's a signal to review readers that the review is incentivized. What companies are doing now is getting reviewers to buy the products to get "Verified Purchaser" status and paying them back via PayPal. That has made it MUCH worse than before, because people assume that a Verified Purchase review is safe.
Having said all of that, Amazon does have its fair share of shady reviewers. Most of the reviewers who were higher ranked than me were still getting free review products after the rule changes. The main difference was that they were no longer disclosing that fact. I could tell who they were because of the products they were reviewing often mapped to offers I was getting from companies wanting me to do the Paypal workaround.
The weirdest thing that happened after the rule change and my incentivized reviews were wiped was that my reviewer ranking went up significantly.
It's the same reason many companies have policies that employees can't accept gifts. In theory they could have a rule "you can accept gives but don't let it affect your decisions", but people aren't robots. You can't turn that off.
Further, having been on the receiving end of a couple of these solicitations myself, they usually took the form of "Hey, we noticed you bought $thing. We'll send you another $thing (or something similar) if you write us a review, good or bad"
Neither of these situations seems sketchy to me, inasmuch as I can't see how someone getting a product for review purposes is likely to taint their review.