Any corporation can break any union simply by refusing to negotiate only for union members. So say the union is fighting for a $1 raise and more pto. The corporation just says "okay, everyone, including non union members, gets a $1 raise and more pto." At that point why would you pay union dues? You can just freeride. To think that corporations won't behave this way is naive, they want to maximize free-riding benefit so that the union crumbles.