I think it is a good news/bad news thing. I'm selling a high-enough number of books to experience a statistical norm of returned books. It's just that this month seems to be the breakout month for returns. Such is my authorial ego that it didn't occur to me that someone might return a book because they didn't like it. I just figured they are fast readers who use Amazon as their lending library.
I guess the real question I should have asked is why allow book returns at all? Books aren't like toasters that either work or don't work. There is always an implied risk that you won't like the book. One of the reasons I price the books at 99 cents is that I realize I am an unknown quantity so I try to minimize the reader's risk by making the price so low.
That said, Amazon is a super operation that knows how to sell books. I would urge any e-book self-publisher to make sure they are on Amazon if they want to get any sales to speak of.