Why should it stand? Messi was clearly offside. The rule of offside is that you can only recieve a forward pass(either through a deliberate pass or an opposition backpass, a deflected pass/shot againts an opposition is not counted as either) if you are behind 2 opposition players. That is one goalkeeper plus one defender or 2 outfield opposition players.
When Pedro(i think) took the shot, Messi was just behind him with no Arsenal defenders between them and Scezny. The moment Messi recieved the pass(via a saved/deflected pedro shot) he became actively offside.