I wasn't talking about proactively ordering one of everything, but ordering something in response to a specific complaint about a specific practice. So, while we're making up numbers, they might actually end up ordering 10 copies of 0.01% of their items. Make the penalties for getting caught onerous enough, and they might deter the practice completely.
Sure, some vendor might then only put a solicitation in 1 in 20 items, but they'll still have a significant chance of getting caught, and it's not like Amazon should be so transparent about the process so a vendor can game it.
But to my original point: it's totally practical for Amazon to take visible action in response to a single complaint, without opening the door up to griefing.
The reason for this is that they don't need to rely 100% on crowdsourced feedback for compliance enforcement (and to think they would shows a really blinkered webtech-centric viewpoint). The complaint -> investigation -> substantiation -> enforcement process is tried and true technology that's even older than UNIX.
I make two fake companies selling the same product. Each of them asks buyers to write a fake review 50% of the time. One of the companies should pass the Amazon inspection.
Well, then you say Amazon can place more than one order for each product. I just keep tweaking the numbers. I'll have 2% of customers for each of my fake companies get asked to write a review. One of them will not get flagged and the fake reviews will help them rise to the top.