Scientific American has a more detailed simple take...
http://blogs.scientificamerican.com/...ows-about-you/
Just like the U.S Postal Service photographing (digitizing and storing) the outside of every letter and package, email headers are Text based and have sender, addressees, and routing info.
Add: There was software developed like Immersion sold to the gov ten years ago, replacing most analysts that used the roll of paper. One would reasonably expect it is more advanced as well as linked with all data sources available. Traditional Americans should expect data collection such as this to require a specific warrant.
The facts are what they are, not what we wish them to be.