While speaking a common language usually (but not always) helps, being a US citizen is the way more important part of the story.

Police in the United States are used to generally treating everyone (including foreigners) with impunity, while cops in other countries dealing with US citizens are more wary of causing a diplomatic incident. There's also the novelty factor: people from high status countries are seen as interesting, and the more uncommon your presence is there, the more friendly interest officials take.

He's making a mistaken inference by assuming that how the police treat US citizens also reflects how they treat locals.