There are people who lack common sense in every profession, including yours.
As for teachers and unions, some school districts don't give the teacher a choice. My conservative daughter HATES unions, but was told by the administration that the $900 per year dues would be deducted from her salary, regardless.
Indeed. Most professions, however, unlike the teaching profession, do not require years of leftist propaganda classes intended to train common sense out of you.
In most professions, lack of common sense results in lack of success. That's another area where most professions differ from the teaching profession. In the teaching profession, nonsense is considered wisdom, e.g.: muliticulturalism, whole word reading, bilingual education, diversity training, tolerance of same sex attraction training, fuzzy math, bizzaro social studies, Maya Angelou on reading lists, etc., etc., etc.(with apologies to Oscar Hammerstein II).
Because of the wacko mindset in the ed schools, schools have become mere socialist, deconstructionist indoctrination centers.