No, we (the public) are quite ok with this
It's two quite separate issues - the duty of care, and a refund of the ticket. The latter is provided for in both cases, and there is no legal interpretation required (and no court cases).
Qantas has been quite creative, and clever, in its marketing of this. You will notice they say 'if you [the passenger] wish to cancel' (or words to that effect), you can receive a voucher.
This is a voluntary passenger action.
That is quite different to Qantas cancelling a flight.
If for some reason Qantas can't in future provide cash refunds due to cash reserve issues, or other financial reasons, and they have to offer a voucher, I'd expect that voucher at a minimum to guarantee the current fare paid for the ticket turned into a voucher. So if you scored a $5999 business class to London, that $5999 would have to be honoured at a later date, not he $9000 they might be asking due to increased demand.