Explain why any U.S. President would find it acceptable to salute 2 U.S. Marines with a coffee cup in his right hand.

On 23 September 2014, President Obama saluted two U.S. Marines while holding what appeared to be a coffee mug in his right (Saluting) hand. This was done as he was exiting Marine One at the Wall Street landing zone in New York City while en route to the UN.It is normal practice using customs and courtesies to render a salute with a free right hand. If carrying something in your right hand, the object should be moved to your left hand (if free) and then render your salute.
Out of respect, if the President is unable or unwilling to salute for whatever reason, he should just give a proper greeting of the day instead of giving an improper salute to any U.S. Service Member. As our Commander in Chief, I feel the President should have more respect for the position he holds and those who serve.