I don't know what you mean by 'subjective.'

That the judgments of "fair" or "beautiful" don't come from a universal source, but from a particular entity. I have copious evidence that what I consider "beautiful" is different from what some other people consider "beautiful;" I have copious evidence that what I consider "fair" is different from what some other people consider "fair."

'Delicious' is clearly two-place, and ordinary English-language speakers routinely consider it two-place;

It is clear to me that delicious is two-place, but it seems to me that people have to learn that it is two-place, and evidence that it is two-place is often surprising and potentially disgusting. Someone who has not learned through proverbs and experience that "beauty is in the eye of the beholder" and "there's no accounting for taste" would expect that everyone thinks the same things are beautiful and tasty.

But this seems to be false simply given how people conceive of 'fair' and 'delicious'.

There are several asymmetries between them. Deliciousness generally affects one person, and knowing that it varies allows specialization and gains from trade (my apple for your banana!). Fairness generally requires at least two people to be involved, and acknowledging that your concept of fairness does not bind the other person puts you at a disadvantage. Compare Xannon's compromise to Yancy's hardlining.

People thinking that something is objective is not evidence that it is actually objective. Indeed, we have plenty of counterevidence in all the times that people argue over what is fair.

Are you claiming that Eliezer lacks any well-defined concept he's calling 'fairness'?

No? I'm arguing that Eliezer::Fair may be well-defined, but that he has put forward no reason that will convince Zaire that Zaire::Fair should become Eliezer::Fair, just like he has put forward no reason why Zaire::Favorite Color should become Eliezer::Favorite Color.

Are you suggesting that this same method stops being useful or respectable if we switch domains from reasoning about this thing we call 'quantity' to reasoning about this thing we call 'fairness'?

There are lots of possible geometries out there, and mathematicians can productively discuss any set of non-contradictory axioms. But only a narrow subset of those geometries correspond well with the universe that we actually live in; physicists put serious effort into understanding those, and the rest are curiosities.

(I think that also answers your last two questions, but if it doesn't I'll try to elaborate.)