The problem is that many teachers and members of the NEA know that only the COMPETENT educators will see this increase, and that they are not part of that group.
What the NEA doesn't understand is that increased competition will be good for education, and that many teachers will see increased wages.
They understand it all too well. The NEA is not in the business of bettering the lives of competent teachers. They are socialist union thugs, bent on maintaining the wage-slave status of the industry, whilst filling their own deep pockets.
I think the reason is actually a little different. I think the appropriate analogy is to Jesse Jackson, Sharpton, and other professional victimization cheerleaders. If their flock realizes that success is within their reach, based on nothing more than their own skills and hard work, the rabble rousers are out of a job.
Similarly, under a voucher system, teachers would be rewarded for skill at teaching rather than political reliability, union/political involvement, and seniority. The power of the traditional educational establishment to control the content of the kids' "education" (indoctrination) would be at risk. Compare this to the mainstream media whining because people can now get news from the Internet, the content of which the moguls can't contol and sanitize.