If they had constructed a cross on public ground, they might have an argument.
BUT this cross was erected privately, and later the government took over the land as public ground. So essentially the government took over private land with a church memorial--i.e. church land, which it had no right to do.
Crosses are put on the graves of our troops every day -- on public land. We're losing about 1,100 WWII veterans per day now. Many of them ache to have a cross placed at the head of their humble plots. Row after row of crosses in our national military cemeteries cry out for us to remember why those men fought and how they sacrificed. Many of them were praying when they died.
Crosses on public ground are not an establishment of religion, they are a recognition of it. The ACLU has been attempting to prevent the merest glimmer of religious recognition on public land, and they are nearly done with their work.
We can turn it around, but it is going to require a new way of arguing, a new alliance between the secular and the religious in our country. We have to be more intelligent than our foes. They have framed the arguments. It is our turn. This cross can serve as our vanguard -- for some it is Celtic. For others, it is Christian. To many, it is a personal symbol of freedom. But it is most of all a memorial to heroes fallen during the Great War. That's why we should keep it. And if someone wants to erect another cross in memoriam to our fallen in Iraq, why shouldn't they be able to do so?
I know there are arguments on both sides, and I agree with you that the public adoption of this land should have come with an agreement that the cross could never be removed on the basis of the first amendment.