I cannot speak to what symbol might be used to identify a fallen Muslim or Buddhist American soldier, probably because there aren't any. Fallen Jewish soldiers on the other hand, are symbolized with the Star of David.

What better symbol than the Christian cross upon which Jesus sacrificed His life; to memorialize those of our brave service men and women who 'sacrficed their lives' in the defense of freedom. None except atheists can deny that the symbolism thus represented trumps any implication that the government is endorsing Christianity.