I'm surprised that a lot of people think a job action would result in an arrest if it was not "legal".
Think of it this way- a modern strike is meant to be a legally sanctioned job action. The workers aren't "quitting" per se, but the immediate results are exactly the same. The employee does not show up for work (in a complete strike). If the entire employee group decides to quit without NMB approval for a job action, it's irrelevant why they quit or whether they even wish to come back. The company will sue their union representation entity and case law supports the company's case. Two main scenarios can play out here:
1. The employees are not planning on returning. The local and/or national gets sued and the courts find a judgement and the company may get what it wants or, if the union dissolves or becomes dormant, it may not. No worker returns, no contract is applied any longer. Company may or may not survive, depending on circumstances. A typical airline with fold
2. The workers intend to come back to work. Things can get really complicated but in short, case law still supports the company. The company will surely get restitution and this could severely damage the local and/or national. It certainly wouldn't speed up negotiations. It's kinda like getting a flag for delay of game. The "referees" will see it as something that the union caused that was not necessary. I can see this scenario also divided into 2 different outcomes:
A. The company panics and rushes to get the contract done. Then (or while getting the contract done) asks for an injunction and then sues and the contract could get thrown out. I think it eventually would.
B. The company does not panic and immediately goes to the judge and plays it out in court with little or completely without negotiations.