The overring question remains for me: if you facing are a US citizen, with the right to be in the US, what is the point of asking why you are entering your own country?
I can understand asking WHAT you did outside the country, but the first question has undertones of suggesting that said officer could deny the constitutional right of a US citizen to be in the US.
Meanwhile we are told all this paraoia is the "prevent foreigners from harming the country", but in fact at this point the govt is challenging the constitutional right of US citizens to enter their own country? Wow....