I received more proof in the mail that Amazon reviews can't be trusted. See the postcard I received below. I was offered a $20 Amazon gift card for writing a review for a product I recently purchased.
Notice how the back of the postcard specifies to "leaves 5-star and write positive review."
I shredded the postcard and chose to not "leaves 5-star and write positive review" based on the principle that this practice is disingenuous. I'll admit, $20 is substantial. With payouts like this, it's easy to see why people are willing to write incentivized reviews.