The design of the collision model is a conscious one. If you avoid the collision, your plane does not take damage, ever. If you do not avoid the collision, your plane takes damage, always.
Being a client based game, the latencies over the Internet brought about this decision. It is the most fair way to handle collisions.
Any other approach removes control of collisions from the one pilot. As Lusche asked, do you really want to take damage when you clearly avoided a collision? That is what you are asking for in the "if one collides, they all collide" scenario.
If you still no not understand why, please ask for further clarification.