Two F-16 fighter jets intercepted a small private plane that violated restricted airspace Friday as Air Force One was about to depart LAX. (azstarnet.com) More...
How could he have known? Sounds to me like a private pilot flying VFR and not knowing that the President of the United States was departing at that time. Also, the F-16s could have just vectored him away talking to the pilot on UNICOMM instead of forcing him down.
Sounds to me like the folks on the ground wanted to talk to him, as far as the forcedown. As far as the rest of it, looks like Toby & Thrusst have answered you well.