You haven't been paying attention:
from Wikipedia:
"Upon presidential proclamation, which has recently included: the interment of Frank Buckles, the death of Senator Ted Kennedy; the remembrance of the 9/11 attacks, the death of Pope John Paul II, the Space Shuttle Columbia disaster in 2003, the victims of Hurricane Katrina, the Indian Ocean earthquake and tsunami, the deaths of Coretta Scott King and Rosa Parks, Virginia Tech massacre, Fort Hood massacre, the death of Libyan Ambassador Chris Stephens, the 2012 shooting in Aurora Colorado, and Friday's Sandy Hook Elementary School shooting."
Forgot one they flew them at half mass when princess Diana died as well. and i agree they should have been flown at half mass for remembrance of the Massacre that happened at that school.