The unions have everything to do with getting those leftist politicians in office.
My husband was a teacher for 24 years. Every time the NEA/AFT mag came in the mail was like lemon juice in a paper cut, drove me nuts that were forced to contribute to that garbage. Look at what your dues pay for. IIRC, the largest chunk of the pie chart was political activism. The unions invest heavily in getting leftists elected.
The unions are complicit in turning the education system into an indoctrination system. That’s a goal of leftists.
I, and many of my colleagues, quit paying COPE to the union. COPE was a small voluntary additional payment to the union, ostensibly for political lobbying causes for the teachers' benefit. It's probably like your IIRC. Instead, the union squandered our dues on lavish banquets for themselves, junkets to exotic vacation places on the premise they were "studying the education system there" (ha!), paying for busing people to lobbies for criminals like Eric Garner, and many other non-teacher-benefitting causes. So, I halted my COPE payments, as did many other educators. Our union is totally corrupt, I won't argue that. But, they do not make up these progressive curricula. The forcing of this garbage on teachers is on the politicians. The unions don't do very much to back their teachers up, less and less every year. So glad I retired a year ago and got off the no-win merry go round teaching has become in NYC.