But I fail to see what it is.

The cross is not and never has been a universal symbol for a fallen soldier for all the obvious reasons including the one that flew right over Scalia's head.

But there is a long tradition that probably goes back to paleolithic times that soldiers are buried either with or beneath their weapon and/or armor.

The advantage to using that symbol over the cross is that by using contemporaneous weapons/armor you can identify the war being memorialized.

I bet that went over your head too....