Didn't the USSC ('90s) rule teachers' union could not force a teacher to support a pol. & recieve the rebate in dues (PAC fees).
I guess the money they were taking for collective bargaining was still finding it's way to candidates.
According to this site, they can opt out in Washington state. It's not a very useful site but there is an interesting article describing how the NEA was slapped with an $800,000 fine for misappropriation union dues.