Dues, teachers, and unions that always support Democrats
We don't know how the Supreme Court will decide the issue of teachers forced to pay dues. According to news reports, the Court is ready to scrap it:
During more than an hour of oral arguments on Monday, the high court's conservative justices seemed likely to side with a group of California teachers who say those mandatory fees violate the free-speech rights of workers who disagree with a union's positions.
Again, oral arguments do not translate into a ruling, as we remember from all of those predictions about Obamacare.
Beyond constitutional issues, the problem is that the public-sector unions are cash registers for the Democratic Party.
For example, the National Education Association donated 99% to Democrats in 2014. Are you telling me that 99% of educators voted Democrat?
The American Federation of Teachers donated 100% to Democrats.
It was about the same in 2012, when the aforementioned NEA gave 96% to Democrats.
We don't know for sure how teachers voted in 2014 or 2012. Let's assume that most of them voted Democrat. However, my guess is that it was closer to 60-40% than 99-1%.
Therefore, it's easy to understand why so many teachers do not want to be forced to pay dues that only support one political party or won't represent the wishes of its members.