actually that's intended to be permitted by RFC 5377 section 4.2:
4.2. Rights Granted for Quoting from IETF Contributions
There is rough consensus that it is useful to permit quoting without
modification of excerpts from IETF Contributions. Such excerpts may
be of any length and in any context. Translation of quotations is
also to be permitted. All such quotations should be attributed
properly to the IETF and the IETF Contribution from which they are
taken.
You're not permitted to modify the text. You are permitted to use it.
Exactly, and disallowing modifications prevents using the text of an RFC
as a comment in implementations licensed under free software licenses.
For short excerpts, one can use the text anyway and claim "fair use",
but larger excerpts can be useful to quote in comments or documentation
and then there is a problem.
