So having seen a few replays of the disallowed goal, I think I can see why the officials made the decision they did, even if I disagree with it...
It was a poor, mis-hit shot that any keeper worth his salt would have saved. If you watch Foster, he seems to stumble and delay before finally deciding to dive for it and missing. You could make the argument that, although he was static and didn't try to play the ball, Yoshida being where he was in an offside position influenced Foster's judgement and prevented him from making what really should have been a routine save. Therefore he was interfering with play.
It's the only justification I can think of for it.