I fully agree with you and I fear this stance is dictated by misconduct and ambush marketing attempts. Volunteers play definitely a big role in the Olympic Movement and there are at the heart of the Games. I still have regular contacts with the Olympic Family assistants I met while working at the IOC.
On a legal term, however, I wonder how this requirement can be implemented : in some countries, social media posts are considered as "private correspondence"...
I would not like to be in LOCOG's shoes : how can we entice volunteers to support the Olympic Spirit if we prevent them from disseminating their personal experience... Is there something rotten in London 2012 ? To post or not to post, that is the question !