They’re clapping because it’s finally over.
No applause for almost the entire speech. Only applause came near the very end after a half hour of dead silence when Obama mouthed some actual patriotic lines off the teleprompter. (Hey, how’d those get in there) Good on the cadets. The applause following the speech-long silence was a pointed rebuff of everything he’d said up until then. Sharp men and women.
Obama would have been better off if they’d not clapped at all. That way, the Obama flacks could maintain that the silence was academy protocol, a military convention, a show of respect not to interrupt, blah blah. But the cadets stopped em cold.