Often I'll find that all the five star reviews describe a completely different product.

That's the trick where the seller replaces a good product's name, description and picture, which effectively transfers all existing(positive) reviews to a different product. Which only shows how many people don't look at a single review if the score is good enough.

The other trick that Amazon doesn't give a damn about is using the mechanism meant for different product colors and sizes to combine multiple products into a shared entry, which mixes all the reviews together and makes it basically impossible for the customer to see the score for each individual item.

As far as I can tell Amazon's only reaction was to start banning customers who returned too many products. I wonder why.

Steam has done something smart here by displaying both the total score and a recent trend score, so you can see if recent changes has made the game better or worse.