When I worked at PacBell, there were certain "union shops" inside the company. If a non-management person had the misfortune to get assigned to those "shops", they were forced to pay union dues...member or not. They were also subject to being "fined" by the union if they worked during a union strike. It always pissed me off that PacBell management would sign off on that kind of unjust terms in a union contract. Spineless.
I never thought that it was fair to force workers to contribute to all union causes.
It might somehow be legal, but it is real wrong.
There should be a way to opt-out of union mandated projects.