I am unsure if this is intentional or not, but hostile AI of any type will not fire small arms at the NATO Hunter or the AAF Strider.
They will engage with grenade launchers and RPGs, but not with small arms. This occurs on all AI difficulties regardless of enemy faction. Besides armored vehicles, they will only behave this way with the NATO Hunter and AAF Strider.
They will, however, engage the CSAT Ifrit with both small arms, grenade and rocket launchers, which makes it odd that they wouldn't do the same to the Hunter and Strider.