I guess the reason they move at half speed across hills and forests is the same reason they move 4x normal speed along roads (inside own territory and in neutral lands). I would not like to lose the extra movement on roads if that is the cost of normal movement across hills and forests. If they had the scout trait then movement in rough terrain would be fixed and they could still use roads.
It seems the Flying ability does not affect the movement speed across normal terrain that other units can use but does allow movement across water and mountains where other units can't move.
JJ