I don't disagree that 25+% of the US is non-white. Do you disagree with the notion that illegal immigrants to the US (and, by extension, illegal immigrants bearing false documents) are disproportionately non-white?
According to a Pew Hispanic Center report, over 90% of illegal immigrants to the US hail from Mexico, Latin America, or Asia. The populations of those countries are almost entirely non-Caucasian, and the illegal immigrants attempting to enter from those countries are almost entirely non-Caucasian. My point is not to bash non-Caucasian folks or to suggest that they are all criminals or illegals. I am simply trying to explain why a CBP officer might ask a few more citizenship and identity questions of foreign born non-Caucasians than he does of white people with American accents.