Let's not forget that these fake reviews are being paid for. That's clearly better than non-paid-for fake reviews. That is, there's some friction for the malicious actors.

Imagine how much less spam we would have if it cost 1 cent to send an email. Friction can do a lot of good.

This is all to say that it looks like Amazon and others (e.g., Steam) are doing the right thing in requiring a verified purchase in order to leave a review.

But it does seem like there's an easy improvement that Amazon could make: offer amnesty to people who report (after the fact) that they left a fake review. The review would be marked internally as fake (and this would impact rankings, etc.) but the vendor would be none the wiser.