An existing collective bargaining agreement (cba) has expired, and the union and company are unable to reach an agreement on economic issues during negotiations for a new cba. the expired cba has a no-strike clause and states that the terms of the old cba will continue in force as long as the parties are negotiating a new cba. the union goes on strike, and the company refuses to continue negotiations until the union at least agrees that continuing to bargain would not waive the company's claim that the union had illegally struck. eventually, the two sides return to bargaining after the company hires replacement workers. the striking workers offer to return to work, but the company refuses to rehire many of them. in court, the union claims that the company committed an unfair labor practice (ulp) by (1) insisting the strike was illegal and (2) refusing to bargain until the union acknowledged the company's position. why it is very important to the union to establish the company committed a ulp?