In my idea of the hedonic treadmill, one can be on the treadmill but slow it down or even switch it off and cease to run on it. More people in Europe are on the treadmill in the sense that they've maxed out their happiness, but they aren't necessarily running on it. Perhaps of the people who are on the treadmill, more people are actively running on it in the US, driven by the consumerism (essentially addiction) that you point out.