"Since the normal way to read is literally, the burden of proof is on you. Otherwise the default setting is literal."
The normal way to read the BUPERS manual is literally; that does not hold for all written material. The default setting here is allegorical, and the burden is on anyone who wants to prove that it should be understood literally.
If you're really interested, you might try reading this article:
http://www.newadvent.org/cathen/07310a.htm
True. But those things that are not read literally have some evidence there that points you to a figurative reading. The natural way to read is literally.