Not Quite Getting The Idea

What is the purpose of a teacher's union? If they were voluntary, I'd say they can have any purpose they want. But in a state like California, wherein I'm required by law to financially support a union even though I'm not a member, the purpose (and legitimate activities) of a teachers union would be very clear and delineated: to support improvements in Darren's pay, benefits, and working conditions.

This teacher, in a letter to the editor on p. 6 of the CTA's mouthpiece rag, doesn't get it:

I am frankly puzzled that some of our members still do not appreciate the very simple (almost simplistic) fact that there is no us/union without our students, the people about whom we dare to write occasional cover stories. We are fighting as a union, from our local chapters all the way to the national (NEA) level, to keep our students in our public schools for a more equitable and well-rounded educational and human experience.

If that's true, why is the union entitled to my money? They're entitled to it by law because the union is supposed to be looking out for my interests, not students'. I should not be compelled to furnish money to an organization devoted to looking after students' interests. It is this teacher and her beliefs that are "simple (almost simplistic)"--in fact, they're dead wrong.