edit: specifically, that people only give 1 or 5 stars

It's mostly just looking at trends. A few things I've purchased from Amazon come with a little advertising slip that says "if this product is anything less than 5 stars, let us know what we can do to make it better!" I've seen plenty of stories from people complaining that their less than 5 star reviews got removed for X reason (often because it's "dishonest").

Maybe that'll be my next learning exercise: scraping the count of reviews from Amazon to support this hypothesis.

“Netflix settled on “thumbs up/down," which is widely understood to imply that you are training an algorithm to know what you like, [Netflix’ Cameron] Johnson said.

“That simple change led to an over 200% increase” in ratings, Johnson said. The inclusion of a “percent match” number also reinforces the idea that these recommendations are personalized, he added.”