[balls187]

Border guards aren't supposed to have a sense of humor.

Imagine how many non-resident/non-citizens enter the US daily. Now imagine if everyone of them were allowed to jokingly answer these questions. It would make doing their job that much harder.

It's much easier to treat all declarations as serious, rather than try to weed out the jokes. And because of the nature involved investigating, anyone who willing makes false statements are punished.