Why assume they always know ahead of time? I was a good student in high school and a shit student right after, it wasn't the professors fault for lacking a crystal ball on that. A decade later I did my part out of genuine interest to learn and it went great.

Again, weeding does and needs to happen on both sides because there is more than one type of reason students fail to learn. That is in no way the same thing as saying it's never the teacher's fault, it's saying there are reasons to weed out for all that need to participate properly who aren't. No amount of money exchange changes that education is a two way street in effort.