Union contributions may be used for political purposes. But a union member must/can request that he be exempted (or refunded) the portion of the union dues which were used for political activities. This set up is done for the sole purpose to intimidate members who don't want to go along with the union bosses. It is easy to imagine a union boss telling everyone in the shop that Joe doesn't support candidate X...Joe's life has now become extremely difficult. And likely his property and life is in danger from some of the hardcore elements of the union nuts.
It is typical union intimidation tactics. Like the voter card check law Democrats wanted passed. Unions are contemptible. They are thugs who use intimidation, violence and coercion. Unions should be outlawed.