I'm not a member of any union. I'm not a teacher. I don't have enough information to give an informed opinion regarding this matter.
What I know of the teacher union (which granted isn't a lot) is I would predict that their position would be support a teacher's job rather than support the teaching profession (perhaps they a smarter than this, but this is my "public perception" of this institution). Now, you say it is an isolated situation. I say it's part of a pattern. I see it as a result of a philosophy/doctrine/dogma. If a union actually represents large numbers of teachers and most teachers don't want these extreme and even less extreme type of situations from occurring then the union would be wise to "do something" that would curb this type of behavior.

Let me bring in a different topic. Does any teacher's union have a means of hearing any parental complaints regarding the teachers belonging to a union?
I simply don't know. I believe that most other types of unions do have this type of feedback mechanism- so that would make me believe there is, but I'm drawing a blank as far as ever hearing about it.