They fired it in self defense, trying to shoot down a Russian missile. For NATO - as well as in common law - the aggressor party is responsible for any actions taken in self defense.
As an example in common law, suppose you are being robbed at gunpoint. In self defense, you shoot at and slightly wound the robber - but the bullet travels on, and kills an innocent bystander. In such a case, the robber can be charged with felony murder, even though he's not the one who fired the shot.
I don't expect a lot to come of this. Poland may ask Russia to pay reparations for the act, Russia will refuse, and Ukraine will get more military aid. C'est la guerre.
