This is mostly an outdated and now probably overreaching law, written in 1948 when most conversations happened face to face and written memos were more significant/official. An employee using Whatsapp to ask if someone wants to get a drink after work or talk about last night's basketball game isn't exactly sinister, but the interpretation is that all of that needs to be done on the recorded, audited channels.
They could still just talk face to face if they wanted to coordinate crimes, the SEC isn't making every bank employee wear a microphone yet.