It isn't. Lots of plays, in fact most of them, do not tell youthe ending first. But it is not uncommon to do this: you start withthe end of the story, then tell the rest of the story as a kind ofa flashback. It is more common in novels or screenplays than inplays for live theatre, but the idea is that our understanding ofthe development of the story is improved by knowing how it is goingto turn out.