A union is different from a church, club, or being a consumer some place because it is at your job and is part of how you generate income.
Non-union employees get the same pay raises union employees do, a rational actor in a non-compulsory union shop would not join the union because they don't want to pay dues. That is why joining a union is compulsory.
You choose to go to church, clubs or spend money at businesses.
If you're part of a union... chances are you don't have a choice.
The difference is that with the "churches, clubs, businesses" - when they do wrong... people go elsewhere (IE: Ask the Catholic Church).
With Unions? You're forced to stay with the Union if you want to keep the job.
IF the union is "mandatory" or the Church is - I think both of those situations are wrong.
If a union is truly worth the fee... it shouldn't be a the end of a gun (Government/Corporate/etc mandate, forced fees, etc).