Honestly airlines are super risky low ROI businesses with huge CAPEX budgets. To expect shareholders to perform a public good at their own expense is somewhat delusional.
I don't think I'm delusional, I just have a different value set to you. But using your public good perspective: -
A) The airline wouldn't exist unless it had been bailed out by the public
B) The public is a 52% shareholder - albeit a hands off shareholder
C) AirNZ already performs other public goods at it's shareholders' expense. It operates a conservation fund and other charitable programmes and it provides discounted fares to people travelling in difficult personal circumstances (e.g. family bereavement). All of those are public goods.
Note: AirNZ made a $332M normalised profit last year. The cost of the regional networks is $12M per annum which is 3.5% of that normalised profit.