Note first that I didn’t say anything about Asians in my comment. Second, I know plenty of Asian American families, including some with kids who did all the kinds of activities that I refer to–and some that didn’t. Actually, those who were discouraged from doing other activities by their parents were more likely (in my experience) to be required to go home and study and practice musical instruments, as opposed to attend cram schools. So these kids simply weren’t allowed to do a broad range of activities that, in my humble opinion, would have made them better citizens, without significantly (again, in my opinion) harming their standing as good students. I felt really sorry for them. I’ll be honest and say that I never observed this particular issue in non-Asian families, although there are phenomena in other families that also (in my opinion) cause some isolation from the mainstream, like strict restrictions on dating and socializing, overcommitment to sports, and other things.
And I don’t think it’s particularly ignorant to point out that an activity that occupies several hours a day–often right after school–limits a kid’s ability to be involved in other activities.