Because they can agree on a set of morals, and it seems--given the commonality of many morals across widely divergent religions--that they're innate to a large extent. So you can have a consensus morality that comes (largely) from outside the individual simply by virtue of the individual living in a community.

What I'm saying about circularity is that it's not fair to judge the morality of a population by any particular specific set of religious beliefs. For instance, Jains might argue that all Christian societies, and the people in them, are morally degenerate because they allow the farming of animals. Followers of ancient Zoroastrianism might find Jains immoral because they wouldn't eradicate evil species, but would find Christians' lack of environmental preservation also immoral. And so on.

"Only a society based in Jainism can be morally upright in the way that Jainism states it should be" is probably true, but not very interesting. It is this kind of circularity that I was cautioning against (in the case of the U.S., with Christianity in place of Jainism).