It's because this is a land of freedom, which many believe was founded by Christian people on Christian principles, so they believe this country is particular blessed by God and exists to honor God. I'm not saying it is or isn't, was or wasn't, but merely trying to answer your question as to why some people intertwine the two.

Why does Christianity, the belief and institution of christianity, is so entwined in patriotism, the belief in institution of the American state? How did those things come about?