Is it a federal offense for the Feds to go through your mail if they have a warrant? I doubt it. The mob communicated in a way where they didn't care if the line was tapped. They used vague terms "heat", "piece", "hit". It took decades for law enforcement to catch up. Hide in plain sight. The mail is super slow, too. Jimmy Kimmel and Ellen can FaceTime chat and get much more bandwidth. Doing it in public helps them, as you can't really prosecute someone for singing a silly song in front of everyone.
A major purpose of secret societies is to teach you this language. Clothes, color, handshakes, greetings, stances, …