I don’t disagree with you in principle, but are the dues “opt-in” or “opt-out”? Is it taboo to opt-out or are there intangible penalties from peers (social, career)? Do you have to renew your opt-out yearly? Are there differing rules for union members and non-union members who pay agency fees? The law says one thing, but real world abidance is another.
Baisley v IAM (2020) asked the court to compel IAM to remove undue burdens in stopping automatic political donations from being collected. It was ultimately tossed because the case law cited in the plaintiff’s argument was deemed not applicable to private sector business.
To your initial argument, yes, the political money is supposed to be “donated”. But given the proper circumstance, a union could automatically collect political donations with the burden of opting out being placed on the employee (union member or non-union). Are they violating the law? No. Are they making it uncomfortable for you to refuse donating? Yes.