The other thing to bear in mind, is that this was not a ban for OFFINABUS directed towards a referee and being punished as an offence under the Laws of the Game for which a player was dismissed. It was a ban for a breach of FIFA disciplinary code Article 57, namely an "offensive or unsporting" gesture - and as the FIFA decision said, it was not possible to be sure who the gesture was aimed at. Much as we might like to think we know who it was probably aimed at, surely you can't penalise someone based on what you think is the more likely explanation, only what you can be sure of.
The principle is more or less the same as the verdict in the court case over the John Terry/Anton Ferdinand "race-related insult" incident. The judge said that even though he thought Terry's explanation of events was "under the cold light of forensic examination, unlikely," since it was not possible to be 100% sure it was untrue, he felt he had no choice but to find him not guilty.