A union authorization card does not “join” someone to a union, nor is it an irrevocable endorsement of “whatever the union will say in the future.”
A union authorization card is simply a means of demonstrating the 30% employee support required for the NLRB to order an election. That’s it. Unions will generally not hold elections until 60% of employees have signed cards, because a majority vote is required.
From the anti-union National Right to Work Foundation:
“You have a legal right to revoke any union authorization card that you have signed. It is illegal for a union to restrict your right to revoke a union authorization card that you signed.”