I am Christian, which you can see by my little icon thingy, and would like very much for all the people I love to know God. I do not, however, feel like the many demonitions of Christianity should be able to impose their convictions on others.
It greatly saddens me that there are many Christians who show their own intolerance OVER their love and compassion of mankind.
I don't think Chrisitians should exhibit anger when somone else's beliefs don't coincide with their own, but rather live their life as a testimony to their faith.
I think that the Christian faith is best represented by those that ACTUALLY think of Jesus' teachings which were mostly about love, and try to live their lives in the same way.
I would guess that people would say that America SHOULD be a Christian nation because they
1. wish that everyone they love knew God
2. think that if everyone lived a life that coincided with "love thy neighbor as you love thyself" there would be a lot less problems.
That said, I strongly believe that everyone should be able to practice any faith they choose, whether it is a belief in God, Allah, Buddha, the Prophet Muhammad, or the faith that they have in Science, Forensics, Biology etc. I believe that when God gave us free will He made it clear that we should have freedom to choose our religion.
He did not "make" us love Him or obey Him he gave us the choice. He wants us to come to Him willingly, to love and to trust Him. I would figure that Christians that make the statement in the OP do not think of it like this, OR it has to do with the Seperation of church and state.
Just my opinion.