While this might be understandable if the copies were distributed illegally, the cause here appears to be a publisher which decided it simply didn't want to offer a Kindle edition any more. Amazon's response, as posted in the forums:
The Kindle edition books Animal Farm by George Orwell. Published by MobileReference (mobi) & Nineteen Eighty-Four (1984) by George Orwell. Published by MobileReference (mobi) were removed from the Kindle store and are no longer available for purchase. When this occured, your purchases were automatically refunded. You can still locate the books in the Kindle store, but each has a status of not yet available. Although a rarity, publishers can decide to pull their content from the Kindle store
All of which underscores the fact that "buying" a book in the digital realm isn't the same as "ownership" in the real world. As David Pogue at the NYTimes explains: "apparently the publisher changed its mind about offering an electronic edition, and apparently Amazon, whose business lives and dies by publisher happiness, caved. It electronically deleted all books by this author from people’s Kindles and credited their accounts for the price."