I have never taken any notice of the use of the cross in that context and think it is a powerful image.
I always thought it was simply related to the fact that immediately after the war (one) that temporary wooden crosses were placed over the graves of everyone that died regardless of religion until a more permament marker could be produced, that is what I have always thought it symbolised, the fields of poppies and temporary grave markers.
That is also why some of the poppy displays you see are mounted on a simple wooden cross, it is nothing to do with religion as such, but simply shows the simple marker used to identify a fallen hero during WW1 of all religions.
WW1 Wooden grave markers
What happened to the Soldiers in WW1 after they died? - Yahoo! UK & Ireland Answers