Indeed, that is the official position of the Democratic Party. The last thing these unions can afford to let happen is open competition between the public schools that their members have dumbed down and other schools that are free to omit the fads, bureaucracy, and red tape that have ruined the public schools. That leaves more money per pupil in the budget for those students who remain in the public schools. The teachers' unions do all they can to create the same handicaps for the voucher schools, or for homeschooling parents, as already exist in the public schools.
Studies show that such courses do nothing to make teachers more effective in the classroom.
Moreover, students taking such courses have some of the lowest test scores of people in any field. But the whole point of having alternative schools is to get away from the things that are dragging down the regular public schools. Iron-clad tenure for incompetent teachers, and paying good and bad teachers the same, are among these handicaps created by teachers' unions. The more bureaucratic requirements the teachers' unions can get imposed on voucher schools, charter schools, or homeschooling parents, the more frivolous lawsuits they can launch against these competitors, claiming that they did not jump through this hoop or submit the right form in triplicate. Even when these lawsuits have no chance of winning, they accomplish their purpose by draining money and time away from those who are competing with the public schools.