Umm… all they have to do is write a Nostr client to pull from the major relays. They’ll monitor Nostr just like the others.
Discussion
My understanding is anyone can see who is messaging who, but only the sender and receiver or whomever has a copy of their private keys can read the contents of the messages because they are encrypted.
The way I describe it to people is it’s like having a conversation in a crowded noisy bar. Everyone can see who you’re talking to, but they can’t hear what you’re saying.