I think it's because some Nations with higher per-capita emmissions (like Qatar) are teensy and can't do much about their heavy outputs (desalination, aircon take a lot of energy). The US is huge, is visible on the top per-capita list and is also the 2nd largest emitter overall. Given that the US is a first world economy, is extremely consumerist and has a love affair with gas-guzzling cars, it's likely that by simply making better choices, the US could likely cut more % per-capita than Qatar. And given their size, that would have impact on a global scale.

Americans are singled out because of two reasons: the fact that north-Americans [1] like the authors of this piece are incredibly north-America centred and consider north-America and northern-Americans the centre of the universe. Add to this the fact that self-flagellation is seen as a virtuous act in liberal western societies like those in the USA and Canada while criticising other countries and cultures is seen as offensive, especially when the criticism centres around those other cultures developing habits which the west is being criticised for - e.g. increased mobility, increased resource use. This is counterproductive since it exacerbates the problems caused by those habits - e.g. China building hundreds of coal-powered power stations - while it would have been possible to forestall this by developing viable [1] alternatives and making the technology available to those countries which do not have the resources to develop these by themselves.

[1] viable as in providing the same benefits while reducing or eliminating the negative aspects, e.g. nuclear instead of coal-fired power stations

This is more a question for google than HN in my opinion.

Whether it’s the most depends how you count and there are usually small countries that come in higher.

But US is always near the top in the lists.

Among countries with big population numbers, US has among the highest rate of energy use and pollution per person, and by far.