You know, there are MANY different religions that serve in the military. And, not all of them are Christian. Many are Jewish, some are Buddhist, and yes, some are even Muslim.
I don't think the families of people who aren't Christian, be they Islamic or Jewish or something else, would feel it's appropriate for their family member they lost in the military to be represented by a cross which is considered to largely be a Christian symbol.
It would have been better (and shown a bit more respect), if they had just put up small generic markers with the names of the fallen on them, with the symbol of whatever faith they practiced above their name.
Just like what they have at Arlington.