In order to be able to transfer liability for the charge from the unknown driver to the known keeper, the operator has to comply with all the requirements of PoFA. However PoFA does not apply to land that is under statutory control, such as airports, which are covered by statutory byelaws.
As such, land under statutory control means that there can be no keeper liability. As APCOA have no idea who the driver is, unless the Keeper blabs it to them, the have to cancel the PCN.