If you look at the Taft-Hartley Act, it spells it out. To paraphrase, it permits employers and unions to operate under union shop rules which requires all new hires to join the union after a certain period of time (1 year of probation, typically). These same rules say that an employer must fire any employees that have avoided paying dues or assessments necessary to maintain the union membership in the union, even if the employee did not violate any of the employers work rules.