First, not all public school teachers are members of the N.E.A. My daughter teaches high school classes and is not a member nor are all teachers liberals. I know you haven't stated that at all but it seems to be "common knowledge" to some.
As you have already pointed out, rating a teacher's teaching ability based on how well the students do on a test does not take into consideration the ability of the students to learn to begin with. You can't make a silk purse out of a sow's ear, (That's my observation, not my daughter's).
My daughter also informs me that students who do well are usually the students who have parents who are involved in their education, parents who bother to show up at parent/teacher conferences,parents who impose discipline and order in their child's lives, parents who care.
In one of her classes of 30 students last year, four of the girls were pregnant, one of them with her second child. Assuming that half the class were girls, that's 4 of 15 and we may safely guess that at least as many of the boys in that class were involved with getting somebody pregnant.
In addition it's difficult to teach high school level classes to teen agers, many of whom have difficulty reading simple instructions. If a kid can't read by the time he is in high school there are only two choices, either flunk him or pass him on through the system.
There are incompetent teachers to be sure, I am positive that my daughter is not one of them, she is a frustrated teacher.
Back in the 70s I was watching news coverage of a teachers strike in Chicago. One of the reporters asked a picket sign holding teacher what her position was at the school. Her reply was, "I teaches English" Teachers who are incompetent should be fired but not on the basis of some test given to students.