What's the alternative?

Each line pilot makes a snap judgement to an alleged contract violation and refuses to fly that flight/trip.

Said pilot is fired by company.

Union goes through grievance process to get wrongfully pilot reinstated, (process takes 1week, 1month, 1 yr, 1 decade?)

Pilot tries to survive in the interim not knowing how long the process will take.

That whole solution doesn't sound grand, so even at the Majors, Union advice (ALPA, SWAPA, APA, etc) is unless you are being told to break the law, you should fly the dispute and let the union fight for you after the fact. At a major you have 13,000 pilots with 12,994 interpretations of any one section of the contract.