All the talk of items left behind, his choice of reading material, etc, etc, are really irrelevant. The REASON that Mr. Shater was denied boarding by everyone involved was that he became hostile, bullying, and abusive to the Captain, the Police Officers, the Gate Agent, and the SOC Duty Officer. If he had relaxed and been cooperative, the situation would have been resolved and everyone would have been onboard. Air-Rage among US Secret Service Agents (or anyone, especially one that is armed) is completely unacceptable regardless of the reason.
After review of the facts and talking to the people involved, the CEO of American Airlines has completely, fully, and with enthusiam, backed the decision by the Captain and SOC controller.