Earlier quoted context omitted.
I mean, it's not a legal obligation, but it might be a nicer world if we considered it an ethical one.
I think perhaps you mean "moral" rather than "ethical". It is entirely ethical to use free software, even to generate massive profits, without paying anyone anything. That's literally the whole point of software freedom: you're free.
Definition of ethical
1 : of or relating to ethics
// ethical theories
2 : involving or expressing moral approval or disapproval
// ethical judgments
3 : conforming to accepted standards of conduct
// ethical behavior
Definition of moral (Entry 1 of 2)
1a : of or relating to principles of right and wrong in behavior : ETHICAL
// moral judgments
b : expressing or teaching a conception of right behavior
// a moral poem
c : conforming to a standard of right behavior
// took a moral position on the issue though it cost him the nomination