Who can explain this sentence to me? Many thanks.

In order to have some fun, we decided to let Calloway and Flick play the border guards for the fictious country. The two were thrilled, and went to our costumes closet to make it more realistic. The boder guard crossing was set up in Crytal City near the Pentagon. When our team showed up in a van, they found Calloway and Flick sitting at a card table in a vacant lot, wearing fur hats and Polish uniforms. The trainees were unfazed and walked up to the table. "Hello," they said. We'd like permission to enter your country."
"Why do you want to come in?" Calloway asked.
Using the cover they had prepared, they responded, "We'er flea enthusiasts and we're planning on attending your flea festival."
Ever the quick study, Calloway said, "Okay, but our fleas are restricted, so we're going to have to do an inspection. Drop your drawers."
The trainees must have had an idea what was going to happen, because when they dropped their pants they were all wearing American falg underwear.


The above paragraphs are about a cover drill of the CIA. I have some questions: First, Why Calloway wants them to drop their drawers? For fun?
Is there a specific meaning if someone wears an American flag underwear? Is it sort of American humor?