They’re not familiar with the local customs.
When Americans travel, it’s often to see the world’s sights—not to experience the culture. Americans show up for sightseeing without doing much more than the bare minimum amount of research. They concentrate on museum opening times, recommended restaurants, and the best photographic opportunities and not so much on proper local etiquette and rituals.
Americans on the road are constantly being shocked—by the age of famous monuments, by the way locals are acting in a particular circumstance, by the foods they’re presented with, and so much more. And a lot of that is due to a lack of research beforehand.