They can fire, but they don't really have the proper angles do it consistently.
If you're technically inclined, the fix is pretty simple. Navigate to whereveryoursteamfolderlives\Steam\steamapps\workshop\content\32470\1125571106\Data\XML\ICW\Hardpoints_Ground.xml
and increase the fire cone height under <HardPoint Name="HP_Empire_AA_Turret">
<Fire_Cone_Height>180.0</Fire_Cone_Height> will definitely allow firing.