If the NSA wishes to change that rule in the future, it can publicly ask Congress to enact a law making it a federal felony for a government employee or contractor to try to subvert, compromise, or weaken public encryption standards. (That would still allow the NSA to subvert, compromise, or weaken proprietary Chinese or Russian military encryption standards, if it is capable of doing so.)
Until the NSA requests such a federal law -- and it's duly enacted -- it seems folly to encourage the participation of its employees in the IETF process, let alone granting them a position as chair of an encryption working group. Put another way, the NSA's signals intelligence mission has eclipsed its information assurance mission.
Even President Obama's NSA review group that came out with a report this week recommended that the agency "should not" weaken commercial encryption software. Why not a "must not?" p36: http://www.whitehouse.gov/sites/default/files/docs/2013-12-1...