I experienced same sitation. I traveled from AMS-DTW-Iowa. The officer in DTW asked me what my purpose was and how long I stayed. Those were regular questions. I told him that I was on my way to seminor in Iowa for 3 days. (I am a doctor. I graduated college in the United States. I need to take continued education program.) After that he asked me again why I had to go to Iowa. Then I told him same answer. He was not happy and was expressionless. I showed everything (registration of seminor, hotel reservation and my business card) to him.
Then he gave me my passport without saying anything.
I understand that immigration and custom officers need info for safety issue. I respect that.
BUT some officers are so rude. They look so tired and are not motivated. For people who are not frequent travelers , the first impression of the country is totally up to them. I am planning go to ORD in November. I will see.