Affiliation vs. charity

Or are people trying to encourage the production of a public good?  Here is David Sedaris:

"A couple of books ago, I put a tip jar on my signing table and I made over $4,000 on my tour … I told people it was all for me to spend on candy. They were delighted because it’s funny to give money to someone who doesn’t need it. If there had been a beggar outside the bookstore, at the end of the evening, he might have had 75 cents. Whereas at the end of my best evening in Dallas [I had] $530 in tips."

For the pointer I thank Bob Cottrell.