Why Neil Armstrong had to clear US customs on returning from Moon?

On July 24, 1969, Apollo 11 astronauts Neil Armstrong, Buzz Aldrin, and Michael Collins had to fill out customs declaration forms upon returning to Earth at Honolulu Airport. The form humorously listed their route as Cape Kennedy to the Moon and back, with their cargo described as "moon rocks and moon dust samples". This formality was more symbolic than procedural.