Why do you think so many unions make it MANDATORY for workers to become members?
Because if they didn't, they'd have freeloaders who receive the benefits of union protection of their jobs without paying the dues.
And BTW, nowadays, it's a very fortunate union that can negotiate a "closed shop" (mandatory membership) contract.
If a union was useful to a worker, workers would be eager to join instead of being forced to. Being forced to pay for an organization that provides no goods or services to you is the LITERAL DEFINITION of robbery.
First of all, unions do provide goods and services to their members. You may not like what they do, or want what they provide, or be cynical about them in general. But you cannot claim that they don't provide goods and services.
I'm forced to pay taxes. I may not agree with what my government does with the money. But I don't question the fairness of paying my share of taxes.