that serves as a general guideline of all laws. Military as a government entity, is governed by military law first. If this is not the case, then anyone serves in the military can use the constitution to excersice his freedom of speech and disobey a commanding officer.

The reason we need all laws beside constitution, is because constitution is way too high-level. It's not possible to use it to govern all aspects of a citizen's everyday life. Constitution needs to be checked only if there's a debate that a lower-level law is in conflict with the priciples specified by the constitution.

Military, in this case, is a very special entity because its members give up certain constitutional rights to keep the entity functional. Despite that, US military still keep all religions on the same ground, as I pointed out already.