Which is to say, it's only anti-union if people don't actually feel they get value from being in the union. If people do feel they get value from being in the union they'll join the union.
It exposes the union pretty directly to one of the deadliest problems in political science and economics: the free-rider problem.
But if the majority are in a union and those that don’t join reap all the benefits the union gains in contracts how is that fair for those who are supporting the union.
In a "right-to-live" country, you would only have to pay taxes if the presidential candidate you voted for won.