Do they have a moral or ethical obligation to support the society that gave them life?
Should the Australian government have supported them more than say the coal or car industry? Hell yes. This is a massive blunder on the part of the Australian government. One that has been repeated for decades.
Could Australia be in the position to provide as much capital as Atlassian is raising now? Yes! There is a LOT of money flowing around Australia. Just a small percentage of Super (compulsory pension funds) could have been invested. Even R&D tax breaks could have given more. However, VC invested by Australian funds is tiny (less than 1 billion/year).
The fact is Atlassian left Australia to avoid paying tax. They have even admitted it many times. The Australian government should have done more to keep them there. Maybe Atlassian leaving will actually do more good in the long term, because more people will see the missed opportunities. (not holding my breath on that one).