[bsimpson]

Many Americans who grew up speaking English have a similar bias against foreigners. When they hear someone struggle to articulately present a point, they (incorrectly) presume that the speaker's lack of competence with the English language extends also to the matter being discussed (e.g. if a person has a hard time speaking about a topic, he also has a hard time reasoning about that topic).

Growing up, the kid who couldn't put together a sentence in English class probably also had difficulty with other subjects. This can prime you to associate a lack of language skills with a lack of general intelligence; however, in the adult world there are plenty of examples of people who are perfectly competent in their field but struggle with English as a method of communication.