This analogy has some flaws. For one, if you write an encrypted letter, the government (at least not to my knowledge) doesn't claim the right to keep it indefi

This analogy has some flaws. For one, if you write an encrypted letter, the government (at least not to my knowledge) doesn't claim the right to keep it indefinitely just in case you're using it to communicate with your terrorist buddies. You can send letters without return addresses. You don't have to use the recipient's real name when sending a letter. Writing something on the outside of the letter makes it publicly visible, and thus carries no expectation of privacy. Writing an email to your friend is the electronic equivalent of a private one on one conversation, and thus carries an expectation of privacy. If you had wanted the government to know about it, you would have CC'd the NSA. I can see why the government would take this position, as it portrays them in a more favorable light, but it's misdirection more than explanation.