I think in many european and US jurisdictions it's probably also driving the proverbial coach and horses through a person's obligations under the anti-money-laundering regulations. The fact that it's a common practice and that the aircrew in question were not AWARE that it's breaking rules would be no defence - they would still be liable for prosecution.
But also - taking undocumented, uninspectable sealed bags onto an aeroplane just because someone else claims it's cash doesn't sound overly clever to me.
YMMV,
PDR