At least as far as morality goes, the US (I think the entire world) is full of foundationalists. People want to know what the basis of morality is. (This is a philosophical mistake.) And worse still, people who cannot find a basis are liable to think morality does not exist at all.
Religious people say the basis of morality is God. Now, this is kind of meaningless. It doesn't tell us anything about morality, except so far as we have ideas about the nature of God. But those tend to be pretty vague. God is all-good, all-powerful, all-knowing or whatever. Alright, so b/c of this foundation religious people will probably make the horrible mistake of concluding morality is good and true. Oh wait, it is...
Or another way, morality "based on God" can be any morality at all. Thus, belief in some meaningless God could totally nullify the ill effects of moral foundationalism. (In fact, for this purpose, the more meaningless the God the better.)
Anyway, now lets contrast with atheists. Of the ones who manage to believe in morality, there are two main supposed foundations for morality. There is "self-interest", and there is the libertarian non-aggression principle (thou shalt not initiate force or threat of force). Both of these, if taken seriously as the foundation for morality, unlike some vague God, do tend to lead people to some conclusions about what morality says. They are not consistent with just any morality, but only a few. And these few are wrong.
Thus it is that for a foundationalist, failure to believe in God, so that God can *meaninglessly* (not entirely, but the closer the better) get lip-service as the foundation of morality, directly leads to significant moral errors.
This can also be flipped around. Suppose you're a foundationalist who first and foremost believes in morality, and doesn't want to fuck it up by accepting some crap like that the basis of morality is self-interest. Then you would refuse to stop believing in God. Mysticism be damned. Morality is more important.