Even if there is a way to deactivate purchased files, deactivation doesn't always work. An example of what can go wrong using Adobe's DRM: it is possible to borrow a library book with your computer, copy it onto your ereader, disconnect your ereader from your computer, then return the library book with your computer. The book will not function on your computer, but it will still function on your ereader. I would imagine that you could game deactivation on purchases in a similar way if such mechanisms existed. Even if the purchase could be reliably deactivated, people could still strip the DRM from the book before returning it.

There is also the business side to consider. Let's pretend that the store issued refunds, but their contracts with Adobe and publishers didn't allow for returns. (I don't know if this is the case, but it is easy to imagine contract negotiations heading that way because of the nature of digital media.) In that case, the bookseller would have to pay Adobe and the publisher for their product while receiving no revenues from the returned product. It's not like a physical product that they may be able to resell, even if it is discounted.