The point is you can only organize those who want to be organized. The worker has to approach the union because the union cannot, by law, promise any wage increases or benefits. The union cannot randomly approach employees and tell them, we will get you better wages and benefits because that is all up to negotiation and membership votes. They can only promise to get the best contract possible. If the necessary 30% are unhappy, someone will call. Sending cards to former union members is just a way to let them know you are there and willing to work with them.