This was a pun by the way. I was playing on "fair" in the sense of retributive justice (Xannon and Yancy punishing Zaire for being antisocial) as opposed to distributive justice. Sorry if that wasn't clear.

On reflection, it is important that these senses are closely linked... societies probably can't get the distributive part of justice at all unless they are first firm on the retributive part. Close-knit, egalitarian communities do seem to get very nasty about members taking more than their fair share, and don't truck a lot of long-winded, self-serving debate on what constitutes a fair share. (On a wider scale, it is also interesting how much noisier and greedier the super-Zaires of this world have become in recent years, ever since the threat of socialist revolution went the way of the dodo. A few decades back, the rich really did fear reds under the beds lynching them any time soon, with compromises like Keynes and Social Democracy among the results. Not so much these days.)

Lastly, I don't think I (or you) need to ignore Zaire's preferences, any more than those of Xannon or Yancy's. Each of them, presumably, has a individual utility function which increases with the proportion of pie that they personally get. The real difference is that Xannon and Yancy are at least attempting to construct a symmetric joint utility function (one which is invariant under permutations of the variables X, Y and Z.) Whereas Zaire is just trying it on.