They applied to all professions, but not at the same time. Check out this possibly-false-but-accurate set of rules for teachers from 1915: http://www.pchswi.org/archives/schools/teacher_conduct_1915....

Sure these rules applied to every other profession as well. The difference is that they probably stopped applying to other professions 200 years earlier than they stopped applying to teaching.

"Guess which organizations are the biggest proponents of keeping things that way?"

No argument there, I'm not defending the unions, just pointing out that historically schools have been extremely abusive toward their employees. Sure the value are different today, but the psychology, social pressures, and incentives are all the same. So if the protections given to teachers were completely repealed teachers might get screwed over for different things, but there would be much injustice nonetheless. But yeah, obviously I don't agree with making it impossible teachers who are bad at teaching, I just think that if there were no protections at all then firings for social or political reasons would become so commonplace that it would drive talented people away from the field. (Granted, talented people are already kept out of teaching today by the unions and a million other policies, but still.)