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…
Fake reviews are just another form of marketing, like traditional junk mail. Making mail spammers pay postage doesn't seem to have changed the fact that most of the snail mail I get is junk, even in the digital age. Charge reviewers a quarter or a dollar to leave a review. If I was passionate enough -- one way or the other -- to leave a review, I'd pay it! I'll bet it would squelch the fake reviews overnight. Just tune the charge to find an acceptable signal-to-noise ratio.