This is all about money pure and rather simple. If the bags had been through checked and then do not arrive in the final destination then the originating carrier is responsible in full or in part. When not on one world connection or on same ticket then BA or any other carrier for that matter need not thru check bags.
As an aside there is no need for PNR details as the entry to produce the bag tag with onward details is a simple entry that any checkin agent can enter. the bags will then be loaded in specific transfer containers for ease of handling at the midpoint.
They don’t do this as BA, in the event of a short shipment, can trace the bag back to the agent and if the bag should not have been thru checked then they can take action. Not sure of they do or have done, but the fact is they can.
Fear is a strong motivational factor in any business.
It has absolutely nothing to do with security as AAA reconciliation should ensure that bags are only loaded on a flight when the passenger is checked in and boarded. If they fail to board the bags should be removed.
Hiding behind security is much the same as hiding behind health and safety. It sounds plausible but is often entirely spurious.