I lok at the meaning of that assertion on terms of game theory. Producing software is not a zero-sum game. If you model it as something like the prisoner's dilemma between the developer and the public, then proprietary software makers are the defectors: they gain a lot, and the public gains a little. Free software makers are the cooperators: they gain less, but the public gains a lot, and more is gained in total than in the first case.

The public good under that definition is rather clear: the total advantage (not necessary capital) that something provides to the entirety of society.

I can see how choosing to act in a less-good way than another obvious way can be interpreted as unethical.