Riverboat,
The answer lies in the definition of Public Transport. If a third party has paid for the aircraft to fly the sector, then it is PT even if there are physically no pax on board. I agree that lawyers could enrich themselves greatly arguing that the payment was only for the live legs and the positioning is free, but if the customer has been charged for the cost of doing it according to PT rules then we may as well keep our noses clean and save our ammo for arguing with the CAA about flights with the owners.