See my reply below. TL;DR a situation where people can receive different compensation if they're non-union, and can get out of paying union dues if they're non-

See my reply below. TL;DR a situation where people can receive different compensation if they're non-union, and can get out of paying union dues if they're non-union, makes it trivial for management to strangle the union. They can entice people to leave the union until it dies. [EDIT] more generally there's a bunch of game-theoretic stuff that keeps unions from being effective if all participation in them is totally voluntary. Union-busting laws usually aim to disrupt the preconditions necessary for the survival of unions, often by simply forcing them all to be voluntary, probably choosing that tactic because there's a personal liberty argument to be made in its favor. Why these people are so insistent that one particular requirement for working for a given employer (join the relevant union) should be outlawed while all manner of other requirements the employer themselves may have are just fine is left as an exercise for the reader.