http://tools.ietf.org/html/rfc6189#section-11
(which they really should have cited in RFC 7169)
When the IETF was deciding whether to standards-track ZRTP or DTLS-SRTP, one of the decision points was Phil's refusal to remove the disclosure flag from ZRTP. The committee wouldn't consider adopting ZRTP unless the disclosure flag was dropped.
Incidentally, this is also a case of creative patent use. Phil received a patent for some core design elements of ZRTP, then freely licensed the patent as long as you correctly implement the disclosure flag.
