It's not all about winning a championship like the OP stated. It's about what coaches can get the kids a scholarship to play in college. How is that any different than students transferring to other schools for any other reason if the other school has better programs that they're interested in whether it's academics, music, dance, etc.?

Say a kid has great athletic ability and talent but the other kids in his school have little interest in sports and don't want to commit to be better by putting in the effort and time it takes to become really good. Also, say the coaches at that school are just teachers who agree to coach for the bump in pay. You expect that kid to stay at that school out of some sense of school spirit?

Instead of crying about it, the school losing kids should hire better coaches and improve their facilities. Won't happen overnight but they can get there if they want to.