These are fees unions in a raft of jurisdictions are allowed to charge non-union members as a condition of those workers’ right to work at all.  Ostensibly, the fees are for the unions’ labor efforts in negotiating wages, benefits, and working conditions for everyone in the workplace.  The Supreme Court is considering a case, Janus v AFSCME, concerning whether such fees are constitutional.

It’s already the case that

Agency fees already are forbidden from paying for advocacy and other political activity.

Money is fungible, though, and even though agency fees might be barred from the purpose, the existence of the fees allows unions to reallocate equivalent money from other sources to the purpose—making it impossible actually to say that agency fees aren’t being used for the illegal purpose.  After all, if a union has $100, it can’t afford to engage in political activity.  If the union also collects a $10 agency fee from a non-union member, it now has $110, and it can afford to spend $10 on political activity, even if it’s forbidden by dollar bill serial tracking from using the non-union member’s $10 for the purpose.  Those ten bucks went illegally, however indirectly, to the political activity.

Moreover,

…plaintiffs in the Supreme Court case argue that negotiating with public agencies requires taking positions on government’s size and scope, which is a political question, so forcing employees to pay for the negotiations violates the First Amendment.

There’s the crux of the matter.  Agency fees are eminently unconstitutional, but I’m not sanguine that the Court will see it that way.  Both CJ Roberts and J Kennedy have shown themselves unreliable defenders of the Constitution, and the four Liberal Justices are a rock-solid bloc.