They set the force protection condition level for so many installations because it is the Unified Combatant Command whose geographic area of responsibility is in North America.
Force protection can include procedures as basic as checking identification cards at the entrance to an installation and requiring credentials to get inside a building.
For instance, 100% ID checks of all incoming persons are almost always used at FPCON ALPHA+, while armed fence line patrols may be done for two days, then stopped and replaced with anti-surveillance measures to increase randomness and decrease the predictability of defense.
In the 2009 science fiction movie Transformers: Revenge of the Fallen, the U.S. military assumes THREATCON DELTA when they believe that the arrival of multiple Decepticons to Earth is a string of massive terrorist attacks around the world.
In a 2003 episode of the political drama The West Wing, "Red Haven's on Fire," President Bartlet orders military bases in Africa and Europe to go to THREATCON CHARLIE after a suicide bombing.