The U.S. doesn't have traditional outbound passport control - checks are made electronically when you book a plane ticket and the airlines are given a go/no-go to issue a boarding pass.
I must have entered/exited US about a dozen times in the last few years and I always had my passport checked by someone in uniform just before Security, when leaving.
Are these people not from Immigration/Border control?
This is travel document checking by a TSA agent. They don't technically require you to show a passport, but most other acceptable forms of ID are very US-specific.
