MP3 files purchased from Amazon have identifying information within them, likely to catch, if not to actually deter, file sharing. At the beginning of the file is some XML including time and date downloaded, the Amazon user ID, a nonce, the distributer (Amazon.com in this case), and a digital signature (presumably so modification will be detected). Easytag does not display any of this XML data.
My intention was to place an excerpt here, but formatting XML within Blogger is more trouble than it's worth. Just view an MP3 from Amazon within an editor, e.g., emacs.
The upshot is that, since easytag doesn't display these tags, I'll have to write my own filter to do such.