'Protecting one's interests in a region' is not, nor is it a feature of, a defensive action. Due to the repeated US intervention in other sovereign nations, the notion of a 'defense' department controlling matters is questionable.
This petition is merely to restore a more accurate naming convention. Due to the lack of declared war in contemporary US history, an acceptable alternative may be 'Military Action' replacing 'Defense'.
It appears the only purpose of the phrasing is to give the DoD a more friendly, less confrontational demeanor. But really, who is it fooling?