It takes two to keep a secret
Government reporters across the country- covering town boards and county councils - can't relate to this, even a smidgen.
http://www.editorandpublisher.com/eandp/search/article_display.jsp?vnu_content_id=1004004734
It's fine to insist on on-the-record interactions between a reporter and a source, unless there's an agreement.
But that's just it - it takes two to make an agreement. Why do journalists allow sources to unilaterally impose anonymity?
Government reporters know there's no such this as off-the-record at a public meeting. Are these speeches and briefing public in the same sense? Maybe not for briefings, maybe so for the speeches, but that's just a guess.
This letter is not bad, but when will journalists and their news organizations truly stand up for against this abuse of power? This would never fly in one-on-one interviews; this shouldn't be any different. It exists because news people tolerate it.
I know - who wants to be the first to walk out, and miss a story someone else will get? It's a matter of competition vs. integrity.