That is because it is not the function of RFC 2822 to specify requirements
placed on Agents, that being left to other documents (notably RFC 2821).
uh, no. RFC 2822 is concerned with the user agent protocol, and it does
place a few requirements on user agents. RFC 2821 is concerned with
the mail transport protocol, so it places requirements on MTAs.
historically we're reluctant to impose many requirements on user agents
because user requirements and programming environments have always
varied widely, and have also changed considerably over time.
I don't think either 822 or 2822 is clear about the semantics of folding
of header fields. but longstanding practice is that both UAs and MTAs
can change the folding from that supplied by the sender.
Keith