From Slate:
Any citizen has the right to petition the president for redress of grievance, or for that matter to insult him to his face. But the potential number of such people is very large, and you don’t have the right to cut in line by having so much free time that you can set up camp near his drive. Then there is the question of civilian control over the military, which is an authority that one could indeed say should be absolute. The military and its relatives have no extra claim on the chief executive’s ear. Indeed, it might be said that they have less claim than the rest of us, since they have voluntarily sworn an oath to obey and carry out orders. Most presidents in time of war have made an exception in the case of the bereaved—Lincoln’s letter to the mother of two dead Union soldiers (at the time, it was thought that she had lost five sons) is a famous instance—but the job there is one of comfort and reassurance, and this has already been discharged in the Sheehan case. If that stricken mother had been given an audience and had risen up to say that Lincoln had broken his past election pledges and sought a wider and more violent war with the Confederacy, his aides would have been quite right to show her the door and to tell her that she was out of order.
More here.