Odd that you view a kid that takes rigorous academic classes and participates in various extra curricular activities as a kid that needs to be "carved out".
In my eyes those kids are valuable participants at whatever school they attend.
The point is that at some schools...there are a large proportion of kids that won't ever attend or participate in ANYTHING...sometimes even school iteself. Public schools have to count all these kids toward their classification numbers. The amount of these kids at a p/p is going to very small; and, as you had mentioned previously, at many affluent suburban publics this will also be a small number. At some high poverty urban or rural schools...this number is high. This is the difference between public and p/p....not that kids are "recurited" or anything else.
To be clear...I was never a proponent of SF or a class "bump" for p/p's. I would have preferred a "clearninghouse" method of counting enrollment (i.e. kids who want to participate in ANY extracurricular - football, sports, music, student gov, or even those parasitic robotics and chess club kids you despise, would be "counted" as they fill out an inschool clearing house form.). Most schools already have a code of conduct form that extracurricular kids have to complete (or something similar) so its not a hard number to determine. This number would be used for classificaiton. The percentage of kids in the clearinghouse would probably be very similar between Chatard, Zionsville, Brebeuf, Carmel....but drastically different at Elkhart, IPS, South Bend, etc.