The concept of tenure and the wording of the contract work together to prevent one-sided, wrong-headed, or patently false accusations from being used to get rid of someone who really did nothing wrong other than refuse to be a sycophantic lapdog.
And sometimes, it helps people like Rafe Esquith, who otherwise would have been summarily fired.
"There are no suggestions that he has harmed any children. But as many of the great teachers I have written about over the years have told me, if you work hard and show administrators how much better our schools could be if they took their responsibilities seriously, you are going to become a target for abuse." - Jay Matthews.