It's simply deductive logic, many people haven't thought of that, thus I want to bring it up that's all.
One incident, my firm sent 5 people to Detroit for an engagement this winter, upon the point of customs, all three of us only had Driver's licenses, two holding their NEW canadian passport, all in business attire and well-groomed, the two with canadian passport (who doesn't speak very well) were questioned extensively. I, for one, didn't have my passport and showed my DL and was free of questioning? was that random? you think what you want.