Honest answer: because in many states, the law is that a certain vote can force that contract into place, not merely deprivation of labor by the pro-union segment.
The reason for this is simple: government has power because it can use violence. Even democracy works because the majority uses threat of violence to get minority submission to law. (Otherwise, all we have is anarchy/consensus). When a non-near-100% majority of the labor force wants to unionize, it only works if had the power to use force-- its own, or the local/state/federal government. Otherwise, scabs will quite often undermine the union's bargaining position.
Nowhere have I said that anyone's use of force is wrong! In the union debates, people often forget to ask: WHY is the use of force legitimate for the federal govt, but not for the labor pool for a factory? The answer is not obvious, the claim might not be true! "Because George Washington and Thomas Jefferson" is not a valid answer!
