This isn't automatically a good thing. Let me give you an example. I had a very persistent Amazon Marketplace seller who kept deleting and re-posting a listing for a specific mobile phone dock I was trying to find. They were selling a fake phone dock using the image and details of a hard to find one I really wanted. I ordered it several times and they shipped a completely different product - different look, color, brand, it was the cheapest garbage ever. Amazon refunded all my costs each time, and because the seller was a punk and refused to pay return shipping I threw them all away. No money out of my pocket.

The reason they would keep deleting and re-listing the item is because I would leave bad (but factually correct) reviews when I kept trying to find the phone dock and seeing their new listing in Amazon's search results. Their re-listing action caused my reviews to disappear because it was a 'new' product listing with their same fake photo every time. I flagged this repeatedly to Amazon as fraud via chat, email, and telephone and they never took any action. This went on for months, and Amazon never addressed it.

So my point is that by allowing anyone to post a review, you give consumers one more tool to warn each other about fraud and unethical sellers. Yes there are a lot of fake reviews, but there are also legit buyers out there too and we are willing to pester fake sellers for months and cost them a fair amount of losses (postage, abandoned fakes because they don't want them back).

Sellers know they can delete & re-list fakes all day long to disassociate negative feedback for an item , and they know Amazon will never ban them.