LibertineIan Royal Nothing to do with playing the game, it's basic physics. Although in the ref's defence it'll have happened at high speed and at a distance. It's all very well having a still photo of it, but if the ref blinks at the wrong time or isn't focusing on that specific point at that time he wouldn't have seen it. And he may well have thought something odd happened, but he can't be sure it was handball unless he saw it, and if he can't be sure he can't rule it out.
Other than Ba tapping his right arm with his left hand, then shrugging his shoulders with a big grin on his face while he was celebrating. I would have done at that point what the Italian ref did with Klose and ask him point blank if he knocked the ball in with his hand/arm...
Assuming the ref was watching the celebration rather than our players protesting. And I'm not sure you can really rule out a goal you've just given because you see the player who scored shrug and touch his arm.