In Janus v. AFSCME, the Supreme Court is considering: Does it violate the First Amendment for a public employer to require employees to pay union agency fees, which will cover the costs of collective bargaining and contract administration? In Abood v. Detroit Bd. of Ed. (1977), the Court that such compelled agency fees implicate the First Amendment rights of government employees -- but that they are still constitutional because they are justified by "important government interests" in preventing "confusion and conflict" among rival unions, preserving "labor peace," and preventing "free rid[ing]" by employees who get the benefit of union representation but refuse to pay its cost. (Abood also held that such fees would not be constitutional to the extent they pay for ideological advocacy by unions, as opposed to bargaining and contract administration.)
In recent years, conservative Justices (and many conservative and libertarian commentators) have argued that this is wrong, and that agency fees should be categorically unconstitutional. Will Baude and I, though, take the view that there's just no First Amendment problem here in the first place, and have signed on to an amicus brief that so states. Requiring employees to pay money just doesn't violate the First Amendment (though requiring them to say certain things or actually join certain groups might).
Here is one way of thinking about the problem, at least as to agency fees being used for bargaining and administration: There are two sides to collective bargaining -- the employer and the employees. Say that a school board is negotiating with a teacher's union. The school board is represented by Manny, the management lawyer, and people working with him. The union is represented by Libby, the labor lawyer, and people working with her.
How is Manny paid? Through taxpayer funds, including taxes that school board employees (among other taxpayers) are required to pay. The taxes are deducted from employee paychecks, and some portion of them goes to collective bargaining on the school board's side.
Say that Owen, a school employee, objects to this use of money that he is compelled to pay (or objects to taxes altogether) -- maybe he disapproves of the school board's bargaining position, or just dislikes school boards. Does he have a Free Speech Clause basis for refusing to pay those taxes? He does not; indeed, forcing him to pay taxes isn't viewed by the law as at all implicating his Free Speech Clause rights (or his right to freedom of expressive association, which is derivative of the Free Speech Clause), even when those taxes are spent on government speech that's part of the collective bargaining process (or for other government speech).
Now say that the state wants Libby to be paid through agency fees that public employees are required to pay. These fees are deducted from employee paychecks, and they go to collective bargaining on the union's side.
Say that Olivia, a school employee, objects to this use of money that she is compelled to pay (or objects to agency fees altogether) -- maybe she disapproves of the union's bargaining position, or just dislikes unions. Why should she have a Free Speech Clause basis for refusing to pay the fees that pay for union-side bargaining, when Owen doesn't have a Free Speech Clause basis for refusing to pay the taxes that pay for (among other things) government-side bargaining?
I think she shouldn't, which is why I think Abood was mistaken in finding there to be a First Amendment issue present at all, and why those who want to strike down agency fees on First Amendment grounds are mistaken. If my analogy is correct, then both kinds of compelled payments (taxes used to support one side of the bargaining, and fees used to support the other) are equally constitutional.
Now agency fees may well be bad policy. Nothing says that the government must ensuring the compelled funding of both sides of the negotiation -- maybe it should only have compelled funding (through taxes) of the management side. There are certainly policy arguments against public employee unions, as well as policy arguments for such unions. But my point is simply that there's no First Amendment objection to government-compelled funding of both sides to the negotiation, just as there's no First Amendment objection to government-compelled funding of one side (the government side).