Look I think the rule is stupid if I was in charge of FIfa I would never implement it and I also wear a poppy around remembrance.
However...look at it from Fifa's point of view. First of all, yes the poppy itself is worn across all political parties and the soldiers it commemorates were not political, but wars ARE political. The wars we commemorate were all commissioned by politicians for one reason or another which since a war always has to have 2 sides, must have a political element.
Secondly, we in the UK are biased when it comes to this. We love the poppy and think it is a great symbol. I bet though if we looked at some other commemorative symbols across the world we might have a different view, maybe we would even use the word terrorists. Remember, these things are in the eye of the beholder. "One man's terrorist is another man's freedom fighter" and all that. I know from personal experience here in Northern Ireland that some people think the poppy is a disgrace and they wear commemorative symbols to the IRA who they consider their soldiers. Now I think they are wrong but I would say that wouldn't I? Who is more right, me or them? Who judges who is more right?
So taking all that into account I can see why Fifa have a policy to disallow anything like this, even if I don't agree.