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.

In that case, at least he received a gentle warning in the form of a few days in the cooler, as opposed to wandering into the US where his attitude could have easily (depending on his skin color and other factors) have gotten him a sucking chest wound for his trouble.

Best to learn not to argue with people with guns at the border.

(Somewhat tongue-in-cheek here but I actually do think this is the purpose of aggressive border authorities. As someone who traveled around old, pre-EU, pre-Schengen Europe, you used to see it quite a bit there as well. Hard to say if it was intentional or just the product of different training, though.)

I don't really have anything constructive to add to this, but the idea that the government is setting up bullys on the border to educate foreigners on how to deal with its citizens in a way that doesn't end in death, is beyond fucked up.