Coming back (pre Sept 11) from a quick trip to Europe I was given this same treatment by a customs official. I was polite for the first few "odd" questions but I was tiring quickly.
Agent: Why did you come back so soon?
Me: I have a business meeting in Salt Lake City tomorrow.
Agent: Who paid for your ticket?
Me: Look! I am a US Citizen, I have a US Passport, I took a quick trip to Europe and now I'm returning. It doesn't matter who paid for my ticket. Are you going to search my bags or let me through.
Agent: (Frowns) Go ahead.