The justification for this monitoring appears to be this: the US Postal Service mission is to "enforce the laws that defend the nation's mail system from illegal or dangerous use; and ensure public trust in the mail.”
Gosh, I thought their mission was to deliver the mail. How is monitoring social media defending the nation's "mail system from illegal or dangerous use"? "Dangerous" use? So if they determine the mail system is being used for "dangerous" purposes - as determined by them - they can monitor our posts on Twitter? That can't be right. What do we have the FBI for?
And "ensure the public trust in the mail"? This isn't how you do it.