What is ignored is that the ball didn't simply strike his arm. It diverted in a dramatic fashion away from Maguire (who was trodden on incidentally aka fouled) and in the direction of the only other Wolves player in the box. Thus the goal was 'scored' directly from the infringement. It can hardly be regarded as incidental then can it? Indeed this is the kind of incident where the attacker benefits from a straight hand-ball that the law was introduced to prevent.