It hasn't always been the case that only MPs can carry firearms on U.S. military bases. A mere twenty years ago, "gun free zones" made their way to these facilities under the watch of President Bill Clinton.
According to a Washington Times editorial written days after the Nov. 5, 2009 attack on soldiers at Fort Hood, one of Clinton's "first acts upon taking office... was to disarm U.S. soldiers on military bases."
Clinton's actions birthed Army regulations "forbidding military personnel from carrying their personal firearms and making it almost impossible for commanders to issue firearms to soldiers in the U.S. for personal protection."
In other words, thanks to Clinton, citizens who join the military to use guns to defend liberty abroad cannot practice their constitutional right to keep and bear arms while on active duty at home.