See issue 7643 for some technical background.  There are some other interesting issues to read if you seach the tracker for 'splitlines unicode', one of which is an open doc issue. Clearly the docs about this are inadequate.

Basically, though, I think you are correct.  email should not be using splitlines().  It was more or less correct when email was splitting binary data, but even then it wasn't exactly correct per the letter of RFC.

Unfortunately not using splitlines has some performance implications...but then again we haven't done any sort of performance improvement pass on the new email code, so it may well be marginal in the overall scheme of things.