There are not enough people in the US to protect all 'vulnerable' bits, let alone police/military personel - this is particularly true if the attackers are willing to die to perform their mission. In all seriousness, adding the entirety of the military to the police forces would do essentially nothing for security. This is particularly true since the role of the military and the role of the police differ vastly.
In response to your other comment, the National Guard did not walk around with unloaded weapons (at least not around here) - one of them managed to shoot himself while attempting to clear his sidearm after a patrol.
-SV