Only takes 1 to fire in a head on. You can collide with the enemy, while the enemy avoids contact with your plane...in essence, only 1 to have a collision.
If one chose to fire on a HO, that means the other one allowed himself to be in guns range of that aircraft.
If there was a collision that means both pilots put there aircraft in the same spot at the same time. Therefore it was a mistake that could have been avoided.
When people say "he rammed me" it was probably just a collision as well. When your ramming someone your going nose first (most of the time), so why not just fire? And if it was indeed a planned ram then the ramming aircraft most likely ran out of ammo and really wants to kill you, aether way if he would have fired the aircraft that is being rammed would have been dead aether way. So no reason to complain.
Not so in game.
I know sometimes when you fly to close to the other aircraft it tells you "system: you have collided with _______" and someone's wing falls off. If that happen why blame the other pilot? It's not like it's his fault.