Very true. However labor violations are seldomly this clear cut mustache twirling villain you would see in the movies which affects all workers equally. Most often it is some mundane error done by a clerk in the HR department that never gets corrected until—by luck—a well meaning lower manager spots it, and goes to the union to demand a correction including back-pay.
This is the reason why outreach is an important part of the union’s job. These errors exists sporadically and will persist uncorrected while the workforce is uneducated on our rights.