Negligible monetary cost perhaps, but sending an email that begins "Qantas has recently experienced major disruptions to our international network." to all of their international customers - most of who wouldn't have been aware of the issues - would have a "cost".

I'm guessing they expected that the majority of Australia residents were aware of the issues due to the negative press, so sending an email like this would be a positive rather than a negative. Most international QF FF members would not have any idea that there were problems (or more specifically, bad press) and an email like this would do nothing but to alert them, and potentially reduce their chances of flying QF in the future.