Earlier quoted context omitted.
Yeah, saying "I'm not a terrorist" is kind of a line I'd expect to hear to in a tv show or something. Do you tell the cop "Don't worry, I'm not a kidnapper"?
Well when you enter the USA under the visa waiver scheme you sign a document that asks you many stupid questions, one of which is are you a terrorist.
(for the curious who haven't travelled on the visa waiver program: http://www.usvisalawyers.co.uk/article6add1.htm )