This bug was apparently introduced as part of the work from issue 4184 in python 3.2.  My guess, looking at the code, is that the module simply didn't work before that patch, since it would have been attempting to join binary data using a string join (''.join(...)).  Richard says in the issue that he wrote tests, so he probably figured out it wasn't working and "fixed" it.  It looks like there was no final review of his patch (at least not via the tracker...the patch uploaded to the tracker did not include the decode).  Not that a final review would necessarily have caught the bug...

The problem here is backward compatibility.

In terms of the API, it really ought to be producing binary data, and not decoding at all.  But, at the time he wrote the patch the email package couldn't handle binary data (Richard's patch landed in July 2010, binary support in the email package landed in October), so presumably nobody was thinking about binary emails.

I'm really not sure what to do here, I'll have to give it some thought.