This prohibition is and always has been standard protocol for the military. We could not participate in any kind of political activity while in uniform.
http://webcache.googleusercontent.com/search?q=cache:GdxnWO4YjD0J:www.gwu.edu/~action/demconv/dncday4.html+%22color+guard%22+democratic+convention+junior+rotc&cd=1&hl=en&ct=clnk&gl=us
The color guard is not proclaiming support for any political cause...... just presenting the colors. Its like saying the military sponsors the Dallas Cowboys or Jimmie Johson for posting the colors at sporting events.