The threat of firing is the great "Do this or else..." It takes all the powerful people a teacher must deal with and arms each one with a nuclear device.
Give my child the lead in the school play, or else. Stop assigning homework to those kids, or else. Implement these bad practices, or else. Keep quiet about how we are going to spend the taxpayers' money, or else. Forget about the bullying you saw, or else. Don't speak up about administration conduct, or else. Teach these materials even though you know they're wrong, or else. Stop advocating for your students, or else.
Firing simply stops a teacher from doing her job.
The threat of firing coerces her into doing the job poorly.
My parents taught in a small town, and I think this is easier to understand in a small town context, where everyone goes to the public school, and a career teacher may teach every child in the town for a generation or more. I grew up in a town of 8,000, and my father taught math to half the 8th graders in town for about 30 years.
So if you're a Democrat, you're going to get all the Republicans' kids. If you play for South Side, you've got all the Moose Club's players' kids. All the doctors, and lawyers, an professors; the drop-outs, perverts, meth addicts and child abusers, too. They mayor's kid, the principal's kid, the prison warden's kid. Everyone.
Some of these people, and/or their kids will not like you and would like to see you lose your job at some point. It isn't that hard to wrap your head around that teaching is a unique profession.