It is surprising that a shallow and slanted statement like this gets published.  Each  Teacher Union represents all the teachers in their district or region. Each teacher at a school is provided due process and representation by their Union for any disciplinary actions and for most policy decisions. Teachers who 'opt out' receive the benefits of these Union services.  It would be ridiculous for there to be two salary schedules: one for Union teachers and another for non-Union teachers. It is also ridiculous for teachers to struggle with internal divisions between Union teachers and non-Union teachers over evaluations.  The Union dues are very small and are also a tax deduction. Every teacher deserves  representation when facing an administrative hearing. The appellation "bad teacher" makes for a funny movie but in real life this stereotype is shocking and upsetting and sometimes career ending. But, the representation of counsel is a matter of due process and should be supported by the Supreme Court.
    Teacher salaries are low in the US compared to the salaries of front-running educational systems in other nations.  Budgetary cutbacks are typical and the new teachers in every school are often not on salary and work for several years at a per diem rate, or a substitute teachers rate, before landing a salaried position. Local Educational Administrations usually attempt to cut costs by forming a substitute teacher cadre which is paid significantly less than salaried teachers. So, Union efforts to improve teacher salaries, to retain experienced teachers, and to encourage entry-level teachers to join the educational enterprise are very important. Perhaps, improving teacher salaries leads to improved student achievement as proven by other nation's educational reforms.
        There are conflicts within schools over the influence of their Union. Some teachers complain that the Union representative or Chapter Chair is not doing enough or representing them as they wish especially with regard to classroom visits by supervisors, or to how the administrator regulates student discipline, or over how the administrator influences teacher's emotional state during the working day. Each faculty as a group may elect their Chapter Chair and frequently this person is adversarial towards the administration creating an atmosphere of contestation and noncooperation. This is an unfortunate result which may be attributed to the working conditions that teachers face: hostile students.
     The real reason however for this action is because Unions can influence their teachers politically by whom they endorse. The wish of the opposition is to limit Union dues by permitting teachers to opt out which will decrease Union revenue and cause them to spend more time and money persuading teachers to opt in and pay their dues, that is, to refrain from being a free rider and to minimize conflicts between teachers over who is Union and who is not!  Less Union revenue from fewer dues paying members translates in today's Citizens United climate to less political ""free speech"" and therefore less political influence.   Each Teachers Union provides representatives for teachers as a group AND protects each teacher from smaller teacher groups, student groups, and administrators. All of these sub-groupings would prefer not to have to deal with due process procedures in determining who is and who is not an acceptable teacher. A Unions role is providing fairness in all educational  aspects of schooling from curriculum, to discipline, to supervision, to remuneration. Many Union-free districts are also free of the serious problems and issues faced by other districts in terms of student achievement and discipline but these Union-free districts are often staffed by teachers seasoned in the more difficult school districts.