How is Nen even a secret?

Heavens Arena not only has a stadium on the 200s floor that seats thousands to ten thousands of people, but the fights are broadcasted throughout the entire tower as well, so all the normal joes are witnessing the contestants openly using and discussing Nen-- even the commentator speaks freely about it during the fights. People are essentially witnessing some crazy magic and are even learning the terminology, how is that not being spread to the general population? Even if all those spectators had to sign an NDA (the logistics of that already sounds like a nightmare), all it takes is a few snitches for the grapevine to come into work and have people talking about it. And all of that is assuming Heavens Arena is the only place where Nen is openly at work, which I highly doubt.