Fear.... That all you discussed in the cruise or otherwise, may be reported in full detail in tomorrow mornings newspaper! So you could never discuss anything confidential, or marginally illegal, without it possibly becoming public knowledge.
All the safety theories here are perfectly sensible, except the one fact, that once the recording is off the aircraft, it is available to be leaked to all and sundry! Or subpoenaed by the tax department, to disclose that foreign investment you forgot to mention.
And from a practicality point of view, just how many 'investigators' would have to be employed to listen to all these hours of conversations? Never happen. No airline will pay
that much for any little increase in safety.