The current legal basis for monitoring email in the USA is not that the gov't has a legitimate interest in seeing it, but that it's old data left sitting around

The current legal basis for monitoring email in the USA is not that the gov't has a legitimate interest in seeing it, but that it's old data left sitting around, so the owner doesn't have a rational expectation of privacy. If those old messages were still left around, but the bodies were unreadable, the government wouldn't have any basis to demand that they be decrypted, since they've not asserted any authority to it (short of a warrant). To do so would be one heck of a long jump.