Not everyone has healthy kidneys and a bladder. Perhaps at the time he was waiting to board, he didn't need to go.
While that's true for many people, I'd say if it were the case for this guy I'd assume he'd lead with that. And maybe have mentioned it to Delta.
I'm kind of surprised by all the "good little puppet" responses this thread has brought forth.
I am as fed up and annoyed by Air Travel's security theater as the next guy, and more than many. I fly quite a bit, and HATE that I have to waste as much of my life on useless idiotic dictates as I do. That said, the good little scared citizens voted for this crap and deciding to be all special and playing the "federal law doesn't apply to me!!" card once you are on the airplane is the wrong answer, and makes it worse for the rest of us.
For that matter, the "Don't let people walk around and go into the bathroom during taxi, takeoff, and landing rule that the dude in Atlanta blew off is one of the few rules that is actually logical and useful from a safety and security standpoint.