Since ancient times, putting fear into the hearts of your opponents — essentially inspiring ‘shock and awe’ — as a way of weakening them mentally and emotionally have been standard tools of warfare. The photo below (explained in the linked article) would certainly be an excellent example of this practice.
Why this photo should “upset” military officials is beyond me. Frightening your enemies so they put up less resistance and even run away is an old, standard military tactic. It’s even, perhaps, ‘kind’ in the sense that scaring them away may be more merciful than killing them in the field (unless you choose to slaughter them in retreat, of course).
In any case, given the bloody butchery inherent in war, scary masks should be the least of any military’s concerns. Heck, maybe the French should even consider using this as a recruiting poster.