This isn't the first time the US government has intruded on the privacy of the mail. During World War II, mail sent home by soldiers was opened and read by censors to insure no secret war plans were revealed. The big difference is it wasn't done in secret, everybody knew the government was censoring the mail. Every single piece of mail was clearly stamped to show a censor had handled it before it was delivered.
The current NSA postal spying is part of their effort to build comprehensive dossiers on every American in the event they might, one day for some reason, want to arrest someone. And the NSA does this in secret because they last thing they want is for the American people to know how much of their lives are being recorded by the government with the intent of using it against them at some point.
Its a brave, new world we live in and one that Erich Honecker could only dream of.