It probably has something to do with the airport's fee structure for international diversions. If there's no agreement between the airport and the airline, there's a risk the airline will refuse to pay the bill. It seems the airport was simply waiting for the airline to agree to pay the charges because there was no formal agreement in place. Is that so unreasonable? There's at least one other major airport in Australia that has had to write off bad debts caused by airlines that refused to pay various charges. Why shouldn't the airport operator try to protect itself?