The third rule of acquisition, quite obviously. But you seem to be forgetting the sixteenth.

More importantly: you are forgetting the meaning of “acquisition”. The instant the author released the software as free software, it was no longer “theirs”. It became everyone’s. The authorship is from that instant irrelevant.

It was “acquired” by the user in a moral sense at the time it was licensed by the original author (for a payment of $0, chosen by the author), not at the time the licensee began using it to make millions.

The original author chose the moral obligation placed upon the recipients of this gift, and they EXPLICITLY chose zero when they picked the license. To donate money to them later is to contravene their expressed wishes.

You can pretend that “they didn’t really mean that” but they are free to commit any text whatsoever they choose in ./LICENSE, and they chose one that declares THEIR OWN PERSONAL OPINION of the payments morally obligated by use of the software to be zero.