Well, as you said, you aren't an expert on this and apparently haven't ever been briefed by your company's legal team, and presumably have never been in charge of compliance. So your arguments about how the law works are in this situation pretty useless.
Jumping away from that angle though, there's still a lot of issues with what you are presenting. For one thing you keep referring to "authorities" without defining who those authorities are. If you're referring to the company IT, HR, and Compliance officers then it seems like you agree with us that the information should be available to those people. However, since that would be a bit odd with the rest of the context you're speaking about I'm going to assume you mean authorities in some sort of government or law enforcement sense.
The thing is that authorities rarely get involved in most of the cases where this information is needed. Sexual Harassment is not a criminal offense, it's a civil one- people don't go to jail for it, they lose money from it. Outside of taking reports these types of things are rarely investigated by authorities in that sense, and there are remarkably different burdens of proof for each of those. Most companies (and individuals I would imagine) also don't want to make a legal issue out of work ones if they can help it, which means it is often in everyone's best interest to handle certain types of problems in house.
Now, as for the philosophy aspect of things, as long as companies are responsible for managing their own trade secrets, sexual harassment complaints, and security in general then all company property (which includes conversations on company servers and services) are open to that company. This is why I do not sign up for company phone plans (except when I want a separate company phone and phone number), and why my work computer does not have my personal accounts on it.