Not to mention that for counter-intelligence the clues aren't the contents of a message but who is contacting whom, when, how often, and from where, most all of which can be determined from network traffic monitoring even if the contents of the message can't be read. In fact, if someone who shouldn't be contacting someone in China/Middle East/etc is doing so using encrypted data, getting a search warrant should be easier. Requiring back-doors to encryption is just laziness on the part of Law Enforcement.