This is speculation...hopefully it doesn't start anything nasty, and I'm sorry if it does but:
The aim of a terrorist is to incite terror in the general public; terror makes uprooting law and order easy and anarchy ensues. Causing bushfires, in my line of thought, would do the opposite. You can see it on the news already; people, at the idea of the recent bushfires being deliberately lit incites anger, a seething, public outrage and anger directed towards the arsonist(s). Not only that, the public is not hiding in fear, or angry at the government, we are banding together, digging deep to help our fellow Aussies in their time of need. Something like this, appalling and terrifying as it is, brings us closer, knowing that we can rely on ourselves as a society to pull those affected through. Wouldn't this be exactly the opposite aim of a terrorist?