Not necessarily true. If enough people find the union valuable, believe in its aims, and want to participate in it, then the employer can't afford to lose or alienate the union employees, so the union has negotiating power (proportional to the employees that participate). If enough people don't find the union valuable, don't believe in its aims, and don't want to participate in it, then the union will correctly have less negotiating power because it represents fewer people.

People should be free to voluntarily associate with groups that represent them and their interests.