Einstein once said, "I do not share the crusading spirit of the professional atheist whose fervor is mostly due to a painful act of liberation from the fetters of religious indoctrination received in youth." I think it's common to converts, to religiosity, non-religiosity, or otherwise, to take up the flag of their chosen way of life with "fervor". A whole lot of Americans were born elsewhere and came to America by choice. And even those who were born here have parents, grandparents, or great-grandparents who were born somewhere else. It's part of our national narrative! We're a nation of converts, fundamentalists for our way of life.
I count myself as one of those fundamentalists. I think America is great. We're not the the only great country in the world, but we have something special. Despite having such a diversity of ethnicity, ideology, culture, language, national origin, etc, etc, etc... we have our basic national identity, and the ideology of freedom and acceptance that is fundamental to it.
I think maybe that's what you're seeing. A lot of people understand that about America, like you said, but it's rarely made explicit.