One of Antoine's review comments made me realise I hadn't explicitly noted the "why not decode with latin-1?" rationale for the bytes handling. (It did come up in one or more of the myriad python-dev discussions on the topic, I just hadn't noted it here)

The primary reason for supporting ASCII compatible bytes directly is specifically to avoid the encoding and decoding overhead associated with the translation to unicode text. Making that overhead implicit rather than avoiding it altogether would be to quite miss the point of the API change.