My mentor for my MA in American history is not a union member. In fact, he was involved in a lawsuit years ago over the fact that the equivalent of union dues were subtracted from his wages due to the fact that the union negotiated the contracts for all professors, including him, regardless of his position on unions.
He and his compatriots won the lawsuit, and he no longer sees money stolen by the union.
The reason that he won is interesting. Apparently, the court ruled that he did indeed owe the union money for negotiating the contract. However, what the union was taking was the same amount as its regular dues for union members--presumably this includes monies allocated for an assortment of other union functions (e.g. grievances, newsletters, red shirts, etc.).
The court instructed the union to deduct from non-union members' wages only the amount necessary for contract negotiations.
But he pays nothing to the union. Why? Because the union does not want its members to know how much/little is actually required to negotiate their contracts.
Unions are run by crooks, which is why it was so easy for the mafia to infiltrate them decades ago and why unions are among the most ardent supporters of the democratic party.