Look, I agree with you - celebratory gunfire is pretty safe, and if you want to be safer, shoot straight up. Not only will it slow to zero, but you will be the one most likely to be hurt by it.
That said, just because you shoot straight up doesn't mean it will slow to zero.
russellbeattie is still right - the person who shot that plane is partially at blame for the damage to the plane and the injuries of the two people onboard.
(I say partially because from what I gather, neither person was a trained pilot and were joyriding.)
