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…
I don't know what law this is but I'm sure if someone ever got angry enough at Amazon and hit them with a massive lawsuit to enforce the fake review problem, Amazon would probably fix it within a few months. Amazon's pretty proactive and they tend to respond best to lawsuits. I follow lawyers for Amazon sellers who sue or threaten to sue Amazon and/or Amazon sellers in the US.