This is truthfully not a difficult question - thats' why nobody is talking about it.
Military bases in the continental US fall under various types of jurisdictions. Not all bases, and very few US Army bases operate under "exclusive" jurisdiction - where civilian law and response is held outside the base perimeter fence line. Land ownership, military land lease, and civilian access is different at each and every base, no matter the service and no matter the location.
"Jurisdiction" under the UCMJ means the military owns military offenders at any time and any place.
"Jurisdiction" as it applies to land, space, response support, ect... means a completely different thing.
Short of typing all day, see this site for addition and clarifying information specifically about "exclusive, concurrent, and proprietary" jurisdiction:
http://www.globalsecurity.org/military/library/policy/army/fm/19-10/Ch5.htm
Military soldiers who are in garrison on an open base are generally not armed and are in training status, not conducting missions (generally) and therefore, unlike in the movies, soldiers are not running around with weapons all the time. You'll probably find that MPs were on duty guarding equipment and providing some law enforcement missions, but the "911" call would have went through civilian "city" or "county" channels so it's not abnormal for a direct response from civilians before communication channels got the word to MPs.
Thank God civilians assisted, and in this case, got there fast!
The other question... President Carter wrote some laws about being armed in garrison. He also tried to disarm the CIA and Secret Service that protected he and his family... He's the guy you're looking for.
PS - You
ARE allowed to have weapons in base quarters, just not in the dorms/barracks.