[telemachos]

Right, but that difference (if it were real, which I doubt) should cut across the two groups of students (American and non-American).

Or at least, we have no reason to think that "business students are lazier than other students" tracks or causes the difference between Americans and others that the author focuses on.

[randliu]

I'm just saying that all the author is observing is that the students with the most to lose, work the hardest. Foreign students obviously have a lot to lose since they're coming overseas for an education. On the other hand, the American students she interacts with are likely wealthy and don't necessarily need stellar grades to move on to a professional degree program e.g. medical school.