The logic for me is that I could easily break the DRM on the books I bought from Barnes & Noble and then use them in any program or device that supports the ePub standard. There is no need to then also transcode it into a different format, though since I suppose it's just words instead of compressed audio, the loss of quality concern isn't valid here.

And yes, the Nook uses the same Adobe DRM that the Sony products do, along with a few others. So it's possible to buy a book off Sony's store and load it onto a Nook, or vice versa.