it is very unlikely that the use of RUS in the US is influenced in the slightest by church attendance

Not sure how you'd prove this negative, Ron, and I'm really not sure (more to the point) why you want to.

All Jack's saying is that if you randomly sampled citizens of the two countries, you'd be four times as likely to find a church-goer in one than the other. It seems like a pretty salient difference to me. Unless you're arguing that the US folk scene is disproportionately composed of atheists - and even then you'd have to argue that it's even more atheist-heavy, compared to the overall population, than the British folk scene is in comparison to its overall population.