@Joe is right. There is a precedent for selective screening. In the nineties I vacationed a lot in Europe. When I returned to the US one of the questions I was asked (on an entry card) was whether I'd been to the UK and if yes if if I had been on a farm. The issue then was Mad Cow Disease.
They still ask similar questions (6059b)
They also ask if you are a terrorist, crazy, Nazi, etc. (I-94W)