Have never been compensated for a review. (Or a comment on HN or Reddit for that matter).
Because those are irate customers who actually paid for the product and feel ripped off. I agree with OP: far more common are gushing, think pieces about how great this product that they got for free is. Often they disclose this fact but in many cases it seems they don't. The unbridled venality feels pathetic, like monkeys trained to do a dumb trick in return for more peanuts.
The main problem compared to other environments is that reviewers on Amazon have no real accountability: try to sing praises of a bad videogame or movie as a journalist or youtuber and you'll be instantly known as a shill and a sellout.
On Amazon, more than taking a mental note of particularly suspect reviewers, there is really little you can do.
They're yours to keep, although they reserve the right to ask you to send them back within 6 months (never happened with me). You can't give them away (you're expected to dispose/recycle them) and you're not allowed to post reviews of those products in other places, although I've seen that happen.
Not always. I've seen tons of critical two- or three-star reviews that were multiple paragraphs and had pictures detailing the issues with the product. Piss off the right person with a shitty product or even an unfortunate lemon, and you'll get lambasted for it.
And then there's people like me; I don't review Amazon purchases often, maybe one in 20 items, but when I do I write a detailed review because either the item greatly impressed me, or seriously pissed me off. I've never done a compensated review (I wouldn't even know how to start doing that) and the few times a negative review resulted in a seller offering to refund or replace in exchange for five stars and a positive review, I've refused.
Only once did I change a review after interacting with a seller, and I left the entire negative text intact with an addendum stating their customer service was helpful and polite, and I was raising from two stars to three solely because their customer service attitude and promises to improve the product made up for the quality control issues in my mind. I did this without the seller requesting it, but they were greatly appreciative and refunded my purchase without me even asking for it.