Well, for one, all the major movies and tv series are American, so they will present any war, conflict, etc, under American viewpoints, to an international audience (including politically naive, historically ignorant and impressionable young ones).

Even when the plot will cast doubt on the legitimacy of the conflict and the actions of the heroes, those will still be the protagonists (and with the humanizing superiority of at least being able to be morally conflicted about their actions), whereas the nuances of the other side are either hidden altogether, or obscured by some "we're all human after all/both sides are equally at fault" message.