Planetary bombardment is bombarding a planet with no real discrimination so it is suppose to do an inordinate amount of damage to the planets surface. You can use tactical bombardment with FO and it will not damage the planet to the same degree.
Air support craft used to bombard enemy troops also need armour to do well against AA, especially heavy forms of AA, but you also have to understand that heavy AA is very inefficient weapons to begin with in comparison to other ground forces, so them being good against air target seem appropriate.
If you armour your fighter they don't dies as soon as they are hit so you can withdraw them once they are damaged enough, thus you can repair them to live and fight another day.
Step one with using the air-force is to suppress the enemy air defence with heavy bombers... once it is suppressed you can use the more unarmoured lighter bombers to better effect.
Another thing is to not use too big guns when firing in orbital bombardment support, the more damage they do the more likely it is they do environmental damage or destroy the infrastructure of the planet, just as ground troop weapon does.