Every place of work has its requirements. With regard to a labor union, its power is weakened if only a few people join. In fact, it really needs all workers to belong in order to make its work effective. At least when unions began, this was necessary influence over big business.
What many workers fail to realize is that they only reason they have decent wages, any benefits or vacations, or a 40-hour work week is due to the battles fought by labor in past decades and centuries. While unions are sometimes corrupt and short-sighted, this should be remedied by reform. The fact is, many workers gain huge benefits from unionization, and thus it is wrong for some workers to pay for these benefits while others do not. For this reason, it could be justified to force a worker to join a union and contribute to it for its services.