Almost half the things I buy on amazon come with a little postcard thing that tells me I'll get anywhere between $10-$30 back as cash if I leave a 5 star review. If I wasn't in a slightly comfortable place financially, and not so lazy, I would absolutely do this. None of those products deserved a 5 star review, and perhaps 1/3rd were returned by me for poor quality. This is absolutely a real problem. The few review sections where people point this out means I won't buy and I probably saved myself a ton of returns by doing that, but now that is no longer an option.

Amazon is cultivating a lot of bad will with this. I imagine their sellers will claim they don't do this (even when they do), then Amazon feels obligated to pull those reviews to please the sellers and in a conflict between sellers and customers, Amazon will tend to pick sellers in fear of them just moving to Aliexpress and hurting Amazon.

and as others have said, these are OEM's, this is their product and no one else sells it. Amazon is pretty much a sales portal for Chinese manufacturers, some of whom are shady like this, to get into the states with minimum overhead, hassle, and oversight.