Neville,
It's about airport demand management.
The operator of the aircraft is responsible for ground handling not the airport..
Perhaps so, but the airport IS responsible for providing the movement areas, parking areas and passenger handling facilities. Smaller airports such as Canberra have limited space and can't accept too many international diversions. Diversion agreements help the airport to manage the efficient use of its facilities by limiting the number of large aircraft that might turn up with little notice.
And even if they get swamped in a 9-11 type mass diversion what does it matter?
It matters because the airport's scheduled flights can't move if the airport becomes clogged with diverted aircraft.