I have heard this line before "them's the rules..."
I doubt if the rule writers had taken into account the scenario of an open locker / falling objects / potential neck injury (incl. para / quadriplegia).
There is a subtle difference between controlled vs uncontrolled / low-potential vs a high-potential risk.
The risk of falling objects is potentially far higher than the risks resulting from pax / FA flying through the airplane in most ordinary scenarios IMHO.
The law on negligence is based on the rule of forseeability where it could be argued that it is reasonable to forsee the dangers of an open locker with heavy (loaded) objects falling onto the head / neck of people sitting underneath.
Were this to happen and the pax suing the airline (as I am sure you would agree they should), the FAs would fall back on their "rules" but I would bet my 2c on the judges to apply
Lord Atkin's dicta.
The obvious winners would be our "learned friends".
PS: I am
not a lawyer