Because private institutions (atleast all that I am aware of) avoid getting involved in non-school matters. What schools do you know of that would get involved?
Having gone to private schools, my experience has been that they are more likely than public institutions to get involved in matters outside of school, since the behavior of their student body contributes to the external image they are trying to portray.
When I asked what schools do you think would get involved I actually meant "provide names of schools." I am curious to look at the policies they have in place and see what is different from the schools I am familiar with.