I look at the strategy of destroying and capturing a town this way:
The fact that the buildings in the town must be "destroyed" is simply a visual cue to let you know how successful you have been. The reality of the situation is not that you would have to destroy all of the buildings, but instead you would want to wreak enough havoc to make all of the people in the city (it's defenders) either evacuate or go into hiding. This is what the destruction of the individual buildings represent.
Once there are no defenders left the paratroopers are free to march toward the maproom.
Unless I'm 500 yards away from the maproom in my M-16 picking them off as they stumble toward the door.
If you accept this analogy, it makes sense that the cannons and guns on the aircraft can easily be used to take down a town.