The graphics and Saavik's order might be unrelated - after all, the graphic says "Intruder Alert" rather clearly. And it's not an area around the bridge that's being highlighted, apparently, but more like a deck below the bridge. Perhaps an intruder was detected on that deck (a malfunction, or a simulation accidentally left on?), or perhaps that deck reported ready to repel intruders? (The graphic is recycled from ST:TMP where it indicated the intrusion of the V'Ger probe...)
OTOH, we could argue that since these "defensive fields" did zip in stopping Khan from hurting the Enterprise
with his weapons, they provided a defense against something else besides weapons. The terminology would be rather consistent if we decided that defensive fields are forcefields that stop transporters: they would be an intruder control technique, warranting the intruder alert text being visible on the same display.