You are just misunderstanding the time line.
McCain and Palin arrived by SUV, and at that time, people were closer to the airfield.
After the Rally, they walked through the crowds, greeting people, towards the plane, to head to their next stop at 6pm. (I don't remember where.)
By that point, the Secret Service had moved the crowds away from the tarmak, and secured the plane.
Thank you! Interesting how our/my preconceived notions can get in the way of reality! Plus - the order of the photos. Standing on the gantry(?)/stairs at the plane and THEN the photos of them speaking.