@Hoss @WilhelmIII @DemonSixOne @PraxisOfEvil @Nudhul I don't think anyone in a vacuum would have an issue with completely voluntary union membership and participation but you're on your own if you refuse to join, (although still protected by labor laws, ofc). The idea that you can just be able to freeload off an agreement that the union and by extension its members fought to get cannot in good faith be seen as anything other than an attack on the ability of the unions to negotiate for better conditions, hence "right to work" laws are essentially attempts to bust up unions.
Although, all this being said, unions are not a good replacement for actually moral labor laws. A loosely connected group of workers are no match for the combined forces of state and capital. And traditional trade guilds often forced their way into a seat at the table of power, because institutional power and not concessions are how you maintain what is rightfully owed to you.