While I agree with your sentiments on a lack of proper data, there is another hypothesis for foreign students,
perhaps having a higher rate of cheating. One that is not bigoted at all, and applies equally well to a subset of north american students.
Some cultures, India, China, perhaps others, currently place immense value on education. This brings with it immense pressure. While that can lead to a good work ethic, perhaps the immense pressure can also lead to a higher likelihood of cheating. It's not that these students underestimate the risk of getting caught, but more that they may be more afraid of the reaction of their family or peers back home if they fail.
Getting good grades is not exclusively a result of hard work. Often it can be genuinely challenging. For some international students, failing can mean a loss of scholar ship and thus having to leave back home. For students in China, for example, this may mean a loss of freedom, and certainly a big loss of social rank back home.
North American students studying here may occasionally have a lot of pressure from home, but they never risk being denied citizenship, or being relegated to low paying jobs for the rest of their life with no chance of improving.
In some graduate programs, this fact is occasionally taken advantage of by unethical professors, who can get away with treating their international students poorly.