Hopefully not treading on too many toes, but I would hazard, being part of an in-group rather than idealogies that are being used to as a reason to come togethe

Hopefully not treading on too many toes, but I would hazard, being part of an in-group rather than idealogies that are being used to as a reason to come together, are closer to what drives Burning man's economy of sharing, than actual altruism. What you just described is the actual altruism. I'm positing that people don't go to burning man inherently to do good - they do it for themselves, and that includes the giving and sharing, in the support of a community they identify with. Let's leave out a few words to get something more universal: "People don't inherently do good - they do things for themselves, and that includes giving and sharing in the support of a community they identify with."