Ah, that explains it. We're supposed to remember conventions from the Usenet days. I am sad to report that Usenet users are in their 50s and 60s now, so someone will need to pass the torch or this information will be lost to time!

Snarkiness aside, I genuinely appreciate that email formatting was documented somewhere, even if it turns out that signature rules were just a convention and not a MUST in that RFC2646 document (as I would have expected given how forcefully people say it is the correct way)