When you can't even give it away...

My first fiction collection, Just Another Day in Paradise, has been free on Amazon and Smashwords for a couple of months now.  Downloads were extremely brisk at the beginning, and have taken an uptick in the last two weeks for some reason.  It's gratifying. Not as gratifying as it would be if all those people were paying for the book, but gratifying nonetheless.
But then...every once in awhile, someone returns one of the downloads of the book.
That's right, they send it back!
Who does that?  Seriously?  Why not simply delete it from your kindle?  Because it's not as if the customer is going to get a refund.
Maybe it's a perceived value issue? Maybe I need to raise the price?
It's a mystery.