Reference: Working Out Combat, Page 72.

Flyers do not suffer the to hit penalty when attacking troops behind walls, hedges or other defended obstacles. This is because flyers fight by overflying or swooping onto their enemy, and are not held up by walls Or such-like. Although flyers are usually positioned to the front of the unit they are fighting, it is absurd to imagine they come screeching to a halt in mid-air! In fact they continue to swoop over and around the enemy, but for convenience we place the models against the front.

If scenery is designed in such a way that flyers would obviously find it difficult to attack their target then players can agree an appropriate penalty or restriction for themselves.