"But to union officials, the right to an impartial hearing is sacrosanct, to protect teachers from losing their livelihoods because a principal or a student might have an ax to grind."
An impartial hearing -- sounds good to me.
We don't know that all the teachers in that article did those things you said. You're implying that we should fire them and make them leave that career, just because a kid and his friends said they did? Kids can lie, you know.
[EDIT: After reading further in that article, I see some of the teachers did misbehave, but they were punished for it.]
Without unions, teachers can look forwards to as much protection and fairness in the workplace as your average US restaurant server. Get on the wrong side of your boss or his son? Fired. Too many complaints from customers about you in one week? Fired. No proof required.