The amount of dues collected from employees represented by unions is subject to federal and state laws and court rulings. The NLRA allows unions and employers to enter into union-security agreements which require the payment of dues or dues equivalents as a condition of employment.
Federal law allows unions and employers to enter into "union-security" agreements which require all employees in a bargaining unit to become union members and begin paying union dues and fees within 30 days of being hired. Employees may choose not to become union members and pay dues, or opt to pay only that share of dues used directly for representation, such as collective bargaining and contract administration. Known as objectors, they are no longer union members, but are still protected by the contract. Unions are obligated to tell all covered employees about this option, which was created by a Supreme Court ruling and is known as the Beck right.
If you work in a state that bans union-security agreements, (24 states), each employee at a workplace must decide whether or not to join the union and pay dues, even though all workers are protected by the collective bargaining agreement negotiated by the union. The union is still required to represent all workers.
You may object to union membership on religious grounds, but in that case, you must pay an amount equal to dues to a nonreligious charitable organization.