I'm not sure if your conclusion follows your premiss there Shonus. Although to be honest I'm not sure what you mean by "the only places where atheism is the norm are those where it has been inflicted upon the populace with overwhelming force". Do you mean like, the odd communist country where they say "No. You are all atheist"? I guess that is proboly true if that is what you mean, although I doubt it will be true in fifty years.
At any rate, if religion was part of human nature, I would think. That. Well. It wouldn't make sense for Atheism to be growing, and especially for the society you pointed out to exist where people only were the bare minimum religious to exist.
Just like, a conjecture (but at least offered with as much proof as seemingly needed, but at least seem to explain the current state of society) but it seems more likely that people are largely both curious about the unknown (especially the unknown close at hand), and sometimes afraid of it. And when they are completely unable to properly start to understand it they are forced to fill in the gaps with imagination. Which is fair enough, although of course in modern days there's no point to that since we are in fact able to start to properly understand the unknown. Additionally, some people (especially children) might lack a sense of basic cynicism and urge to find the truth, and thus fill in their blanks with ether their own random imagination or whatever they are told.
Both of these tenancies could give rise to religion, but certainly it's not actually a natural inborn impulse to create religion. Unless by religion you mean "Making shit up when you got nothing better" Which I suppose I could agree to. Although comparisons to a appendix come to mind in that case.