Because your employer wants the union to be happy. You also need the customer's permission to work; because if customers ask your employer to fire you, your employer will think that it's a good idea to fire you. edit: TBH, you need my permission to work, too, if I know enough of the customers or have enough followers on my twitter account.
I can assure you my employer would like the union to fuck off.
In a world where it isn't illegal to fire everyone who even thinks of joining the union, you might have some (ethically wrong) point. That's not in this world.
