That's fairly standard. The downside, most construction companies will have many COs (Change Order)to supplement the work.
Just because a bid is lower doesn't mean the subcontractor is hiring illegal labor.
Parent company that accepts bid isn't morally responsible to check on their subcontractors. If parent company is responsible, imagine the possibilities? Ever subcontractor would then blame the parent company for just about everything since they're not held responsible.
Did Tesla realize that labor wasn't "legal" during the process? Probably...but what now? Stop building the factory? They have investors that don't care about morals or ethics.