I would guess that it's to avoid the case where a server implementation always rejects any request for an authzid.
Well, I have this problem with an XMPP server, but in XMPP, the authzid for a user is different from its authid, so the check doesn't help. Luckily, an authzid in XMPP can never be used as an authid, so we don't have a problem with the equality check. cheers, Remko
