It's the political element to this which has resulted in FIFA not allowing the poppy on the jersey. While the poppy is seen by you (and many others, granted) as a universal symbol of remembrance, the reality is that it is worn by many people as a symbol to remember "our" war dead. I say "our" as in the person wearing the poppy.
There is a large section of the world who will see the poppy as a purely British/Commonwealth symbol, and its alignment with the Royal British Legion strengthens that perception.
This is on their website..
"Given their lives for the freedom we enjoy today"
That's a political statement, no matter how much we all agree with it.
The funds raised from the sale of poppies doesn't go to the soldiers or families of Iraqi dead from the two gulf wars. It doesn't go to the Argentine veterans of the Falklands war, nor does it go to German, Russian, Japanese veterans etc. This is why FIFA have taken the stance they have.