Why is so much real policy debate done behind closed doors? Joseph Stiglitz's "The Private Uses of Public Interests" (PDF) argues that sometimes it's for security... but most of the time it's to protect private interests:

The one argument that may have some merit is that hiding information may sometimes provide a tactical advantage in the political bargaining game. But my own experience is that all too often, secrecy is neither justified by national security interests, nor as a prerequisite for rational and thoughtful debate, nor even as a tactical necessity in a broader strategy, but rather, secrecy serves as a cloak behind which special interests can most effectively advance their interests, outside of public scrutiny. There is an old expression that sunshine is the most powerful antiseptic. In this sense, I understood why discussions concerning privatizing the production of enriched uranium-the critical ingredient of nuclear bombs-had to proceed in secrecy. It was not because national security would have been jeopardized, but because there rightly might have been a public outcry if it was known that we wererisking nuclear proliferation for at most a meager few hundred million dollars. I also understood why discussions concerning ethanol had to be conducted in secret-again, private interests seeking favorable treatment might have might have failed to get what they wanted had there been an open public discussion, especially amidst accusations that campaign contributions seemed to affect public policy.