Big Lizards runs through several potential explanations for the silent treatment, then offers this -- the soldiers were ordered to be at the speech, thus ruining a lot of plans for the holiday weekend:
the holiday reunions for many soldiers and their families were ruined. Gone are their (possibly non-refundable) flights and difficult to get hotel reservations; gone also is half the time they expected. Not to mention all the wasted money they couldn't afford to lose. All gone.
And for what? Because of a military emergency? A hurricane or other natural disaster? For what lofty reason were these soldiers forced to donate their time, the most precious resource for an active-duty soldier? To listen to a classless, clueless politician talk about how concerned he is about military personnel... while using them as a prop in his own reelection campaign, without regard to whether they want to become part of his next campaign commercial, or even whether they support his reelection in the first place.
I'd say the poker-faced silent treatment Obama received was no more than he deserves. The brass can order the troops to assemble and be respectful to the Commander in Chief, but they cannot order them to cheer spontaneously.