>what good have they done?
For most teacher's it has nothing to do with politics.
It's the only way to get legal backing when you need help against false accusations by students or railroading by power hungry administrators. You're out of a job the second a student accuses you of misconduct unless you have lawyers with big sticks ready to move in.
Only game in town, insurance-wise, against real world legal shit. You're fucked otherwise.
There would be asshole teachers, unions or not. Who gives a fuck about them?