I suppose this will sound like I am stirring the pot now, but if Mohammed Derka Derka or Jaime del Coca get stopped, answer "Yes, I am an American citizen," and are then free to go, why have the damn check in the first place? Seriously, sounds more and more like an expensive farce.
I'd be more impressed if the Agent said, "Sir, step out of the car now and show me your ID, or my colleagues and I will forcibly remove you and charge you with a felony." But I suppose they don't have that power, for a variety of good reasons. (Are citizens required to carry proof of citizenship within X miles of the border? Do you have to demonstrate citizenship to get a driver's license in border States? How about if Oklahomans fly in to a border town and rent a car?)