Basically the big difference is that the decision to abort when you're about to run out of air is pretty black and white, deciding to enter an overhead environment is much more of a grey-area.
REALLY???
So then the newbie diver and the "cave" scenerio is a "grey area"? So why is the DM being taken to task? If the 'newbie diver' in question shared your view that it as a grey area then I don't see ANY culpability on the part of the DM... if the newbie diver saw this as a clear no-no... then there is no difference between the air scenario and the overhead scenario... again... no culpability on the part of the DM.