[jrockway]

No, the DHS treats citizens like that too. My understanding is that it's purely a psychological technique to extract more truthful stories. People have plenty of time on the plane to practice their cock and bull stories, but when some key detail changes during their interview, they aren't so good at thinking on their feet and they end up being caught in a lie. "This picture doesn't look like you" is the oldest trick in the book. If the customs agent really thought the picture didn't look like you, he wouldn't have told you. You would have been led away without question. What he really wants to do is get you on the defensive where you're less likely to remember your pre-rehearsed story.

(If you ever want to watch Customs in action for four hours, take the train between the US and Canada.)