The owner told me that her business was divided between kids needing extra help and kids who are ahead of grade level.
I expect you're right that some of the parents are pushing their kids. That said, the owner told me that she has one kid doing algebra in 3rd or 4th grade, and another who's doing algebra in 5th grade. No amount of pushing would get those two kids to algebra at that age.
I wonder if some people use it to replace poor instruction in regular classes. For example, at my kids' school they use "reform" mathematics (aka spiral mathematics). Last year the 1st grades cut out pictures of triangles from magazines ("geometry") and copied the names of presidents on coins (???). Argh. So I have a deal with my 2nd grader: I do his pointless math homework and he does work at his level at home. Today I wrote a two sentence story about lions. This was "mathematics." I am not making this up.
We're thinking of trying Kumon. My sons love maths and are excited about the contests and prizes.
