A union's primary purpose for existing is to promote job security and higher pay and benefits for its members. That is certainly true of the teachers' unions even though they give a lot of lip service to caring for kids. In fact, there is evidence which suggests to me that they could care less about kids.
A case in point was the recently defeated school voucher proposal passed by the Utah legislature. It passed the Utah legislature but was placed on the ballot in the recent November election. It would have given low income students a voucher worth between $500 to $3,000 dollars to attend a private school of their choice, much less than the $7,500 it costs Utah taxpayers to educate a public school child. George Will reported prior to the vote that millions of dollars was being spent by teachers' unions across the country to defeat the Utah referendum. He noted reports that the NEA approved expenditures of up to $3 million to defeat the proposal and state teachers' unions in Maine, Colorado, Arizona and Wyoming were also contributing to the campaign to defeat the new law.
While I prefer the use of tax credits and tax incentives for private scholarship funds rather than vouchers, the unions certainly didn't argue that they preferred other ways of empowering parents. Their fear is if low income parents have a choice they will leave their public schools. That's reminiscent of the Berlin Wall mentality. The wall was built to keep people from leaving the communist regime not keep people out. In the same way, the unions oppose anything which threatens their monopoly of power over public education. The result is the education of kids, the primary goal of public education, is lost in their drive for power and money.
I believe truth and right ultimately prevail in life though it often times takes much longer than I would prefer. I think the same will be true with meeting the education needs of parents and kids rather than the power and money desires of the teachers' unions.