I assume the air-crafts were expensive US property. Were USAF and Navy personnel ever punished for this? Was there a request process to do this? Did commanding officers turn a blind eye?
Also when did this stop in the US military? Link for reference.
EDIT: found this on the site I linked The female pin-up occurred in various stages of dress (often undress). Lack of restraint helped foster the imagination of the artists and aircrew and the sexy pin-ups reflected this wild abandon. As a result, the Army Air Force unsuccessfully tried to restore a sense of decorum with, AAF Regulation 35-22 in August 1944.3 This regulation allowed nose art, but tried unsuccessfully, to institute a "sense of decency." Pin-up nose art, however, was the exception on the Navy and Marine aircraft because of the directives prohibiting nose art.
I would still appreciate more insight!
Aircraft might be expensive, but a coat of paint is cheap. Nose Arts could serve as good luck charms, morale boosts, etc.
Morale. Air crews suffered a far higher casualty rate than infantry, I think the highest in the services. It was a great morale boost to let the guys plan and paint the nose art.