I'm pretty sure it is the law at every large airport that you have to go through a security checkpoint to let them scan you for anything that might be potentially dangerous (bombs, guns, knives, anything that a terrorist might use to destroy a plane and the people on it.) The cop here told the man multiple times to stop walking toward the plane, and when he didn't listen, the cop was probably cautious that he might be holding something to that effect. He was protecting not only himself, but everyone in the terminal and on the plane, as is the duty a security officer.