From Scott Kozel:
(Stuf4) wrote:

The Hague Rules of Air Warfare
The Hague, December, 1922-February, 1923

http://lawofwar.org/hague_rules_of_air_warfare.htm

Excerpts:

CHAPTER I-Applicability: Classification and Marks.

ARTICLE III
A military aircraft shall bear an external mark indicating its nation;
and military character.

[Note: There are no external markings on military shuttle missions
that indicate the military character of its missions (-the original
point in question-).]

A space shuttle is an 'aircraft' for relatively brief portions of its
mission, and then only for ascent-to-orbit and descent-from-orbit.

Its actual mission is carried out in space, where "Rules of Air Warfare"
and rules for "military aircraft" do not apply to a spacecraft.

Examples snipped from that post showed how the US Air Force complies
with the military insignia requirements specified in these Rules of
Air Warfare.

NASA doesn't.


Other examples from the X-15, X-20, ICBMs, etc can be examined as
well. If the Air Force agreed with your line of reasoning, they too
could abstain from their use of military markings. But these vehicles
are clearly marked in accordance with the Hague standard.


~ CT