That's the whole point of sampling. If you get a big enough sample, and you have a reasonably fair way of getting it, then all those things wash out as individual variation.
I agree. But it is very rare that I see a study that samples people living in more than one country, not to speak of sampling of people from all countries in the world. My point is this: If a study samples only people from the US, it should be highlighted very early (like in the first sentence) that it is only valid(ated) in the US and cannot be applied elsewhere, without testing.
