An employment contract can't violate labor laws and a judge will not look kindly on a person taking unjustified legal action (like threatening to involve law enforcement) based on a contract that in itself might be full of unenforceable clauses. Further the types of workers that have stipulations against striking are generally those holding jobs pretty critical to vulnerable people: air traffic controllers, nurses, teachers and EMS services. A bunch of scaffolding workers going on strike is going to result in dangerous conditions? A construction project taking a couple more months to finish hardly qualifies especially when project delays happen all the time for numerous reasons. Furthermore these workers aren't even going on strike, they're simply refusing to work overtime.