The point is liability, plausibly deniability, etc.

If a trader uses their personal cell / WhatsApp / etc in today's age, all the liability falls on that trader.

If organize drinks with a coworker adhoc and communicate things they shouldn't.. the same.

However, if a trader on their work PC, on the work network, in the office can communicate in electronic form on an unsecured, unmonitored channel.. the bank is paying a very large fine. This is like leaving gmail unblocked on work computer/network.

Likewise if the bank itself organizes a monthly drinks between 2 departments on different side of the Chinese wall, offsite and things get communicated that shouldn't .. its obviously somewhat intentional by the bank and they will get fined.