For all their lofty rhetoric, educator unions do not exist to serve the children; they exist to serve their members. And as you can learn in any collegiate organizational behavior course, all organizations, including unions, take on a life of their own and seek to expand and propagate themselves. Today’s educator unions have departed from even serving their rank-and-file members to merely serving themselves. They have become powerful lobbying groups that promote bleeding-edge leftist policies and that serve their officers and staff.
My oldest sister was a teacher in the Clark County School District for five years, and she used to complain about the union. She was a member (it was practically required), but she mildly resented the money that went for dues, and she certainly didn't think they represented her interests.
Thanks to educator overrepresentation in our education system (and probably due to a lot of parental apathy), schools have become increasingly bureaucratic and are hamstrung by policies that prevent implementation of improved teaching methods. Teachers are just as much hostages to this system as are students and parents. They are often not even able to consider better teaching methods due to heavy-handed union contracts.
In addition to not being "able to consider better teaching methods," good teachers are often forced to implement new "better" programs every few years, at considerable cost to the schools and considerable time for the teachers. When my sister first began teaching at a particular Las Vegas elementary school, she was taught to use a certain reading program. Within three years, this program was replaced by another reading program, which required the teachers to be trained and drilled on the techniques to be used. My sister's main interest (and therefore studies) is in early childhood reading instruction, and she did not think that either program was that spectacular. However, true to the contracts and requirements of the school district, she could not deviate in the slightest from the reading program rules. She could not choose to use a different, possibly better way to teach her students. And that, in my opinion, is a waste--of the school district's money (in licensing the reading program), my sister's expertise, and of the students' time in the classroom.