I really doubt that's actually happening. For a long time there was a Vine forum, and the culture was definitely about reviewing as honestly as possible. I'd posted many positive but also many negative reviews before being selected for Vine.

It also seems like there are many more products needing reviews than reviewers. I've never worried about being negative.

And finally, the item is definitely not free:

* Amazon displays the tax value of each item we're offered for review, and reports it to the IRS if we choose it. So literally, the product is taxable income.

* Some items, like books, take a lot more time and effort, compared to their monetary value. But all items require time.

* Some items have collateral costs. Just a couple of examples: I once reviewed an AC/DC converter for the car which blew a fuse when I tried it out. That cost me a trip to the repair shop. Another time I reviewed a floor lamp which arrived destroyed in the box and left my living room speckled in glass shards and styrofoam pieces.

tldr; To me, it's a contractual relationship that I'll continue doing as long as it makes sense to me.