I think of this in reverse. It's legal for the government to track mail - who sent a message, and who it's going to. They have access to the "outside of the envelope". But it's not legal for them to read the message inside.

And this same principle allows them to build massive friend/connection networks of everyone electronically. The government knows every single person you've communicated with and how often you communicate with them.

It was never designed for this originally.