You are a new teacher and you are told that whether or not you opt in to the union, the union dues will be subtracted from your paycheck (you can opt to donate this money to the Red Cross if you want, but you can't keep it)
You don't agree with all the union politics, but you know that there is a certain liability involved in teaching. As you talk with more experienced teachers, you learn that even the very best, most professional ones have been threatened with a lawsuit or a formal complaint at some point in their career.
You can look for "malpractice" insurance privately, but it's not commonly sold to teachers, is expensive... and you'd be paying for that AND union dues.
If you join the union, you get protection from potential lawsuits.
This is why I joined the union. I am not particularly political, but I'm a realist. If I'm paying for the service anyway I might as well join and get the benefits. I think most teachers in APT/CTA join for the same reason. Most of us just want to do our jobs and don't have a big agenda to milk the state budget with our gigantic pensions or bankrupt the district with our greedy step and column raises, I think most people in the private sector would make the same choice if they were given the same options.