Union contracts leave things less open to interpretation, union advice helps employees understand that this is a legal issue, and coordinated legal action and representation help you win the fight in court.

Leaving it to individual employees to individually see the problem, recognise they have legal standing, then fight Phoney Stark's infestation of lawyers isn't super-realistic, and turns the employment contracts into nothing more than a weapon to fuck with the employees, because Musk can just ignore anything in the contract favourable to employees.