It's absolutely wild to me that anyone has ever operated under any other assumption.

Maybe because they're aware that complaining about the boss is protected by law (in the United States and many other countries).

It being protected has nothing to do with a presumption of privacy in corporate communications. At a minimum you should be aware that your work related communications are subject to discovery.

It amazes me that people seem to think that once they have clocked in for work they have entered some kind of dystopian dictatorship where all their rights are immediately forfeited. And that people are fundamentally not allowed to push back against this kind of bullshit.