Military Freedom(?) of Speech

I'm not sure why Admiral Mullen felt compelled to write this warning. But I know that the fact that he wrote it unnerves me somewhat.

The highest-ranking U.S. military officer has written an unusual open letter to all those in uniform, warning them to stay out of politics as the United States approaches a presidential election in which the wars in Iraq and Afghanistan will be a central, and certainly divisive, issue.

"The U.S. military must remain apolitical at all times," wrote Admiral Mike Mullen, chairman of the Joint Chiefs of Staff. "It is and must always be a neutral instrument of the state, no matter which party holds sway."

Is it just me, or is Admiral Mullen afraid of something beyond the scope of routine candidate support from among the troops? Like the writer said, this is a most unusual step by the JCS. I certainly don't recall ever seeing anything like this before.

How does one interpret this? Is the Admiral telling the troops that they cannot engage in Constitutionally protected activities (as one would surely be inclined to think given the wording of his warning)? Or...and I'm just pulling this out of my nether regions...is he afraid of what might happen to military funding if President Obama rightly perceives that the military rank-and-file are more disposed to Senator McCommie than to him? I'm just saying....