Here's an analysis from a couple years ago: http://minimaxir.com/2014/06/reviewing-reviews/

In particular, the conclusion:

"The reviews on Amazon’s Electronics products very frequently rate the product 4 or 5 stars, and such reviews are almost always considered helpful. 1-stars are used to signify disapproval, and 2-star and 3-stars reviews have no significant impact at all. If that’s the case, then what’s the point of having a 5 star ranking system at all if the vast majority of reviewers favor the product? Would Amazon benefit if they made review ratings a binary like/dislike?"

I remember a talk by Thorsten Joachims at ECML/PKDD 2013 where he was talking about this, you can watch it here.

https://www.youtube.com/watch?v=fX9lj0UdB9s

Around 11:40 he shows evidence of this "dishonest" behavior. As far as I remember, the whole talk was very good. He has some publications on the topic.

I am having a hard time digging it up, but I remember reading some reporting on the Netflix Prize that said (before Netflix abandoned the star system) that many users rated things only one-star or five-star.

But, counter to the OP's point, I wouldn't assume this is an attempt to move the average; I would guess this is for a number of reasons, including because it's too much mental energy to decide if a product (film) is worth four or five stars, if you rate something you are often just trying to say "liked it" or "didn't like it."