By the way, just so we're clear here (since 'surprise' SocialistSlayer didn't include this), the ban is the result of the Posse Comitatus Act of 1878, which prohibits the Government from using the military for domestic law enforcement. And let's not pretend that Texas just didn't go full retard over the US Military's exercises in Texas thinking that Obama was invading (Yes, I know, it's kind of hard to invade a place you already control...but...Texas). So, the US Government and Military are quite sensitive about having armed military personnel in within the US.
Additionally:
Military officials said security at recruiting and reserve centers will be reviewed, but the Army's top officer said it's too early to say whether the facilities should have security guards or other increased protection.
"I think we have to be careful about over-arming ourselves, and I'm not talking about where you end up attacking each other," Gen. Ray Odierno, chief of staff of the Army, told reporters. Instead, he said, it's more about "accidental discharges and everything else that goes along with having weapons that are loaded that causes injuries."
Source
So, the Army Chief of Staff disagrees with arming soldiers inside the US (Obviously referring to off-base activity). Go figure.