Well, duh. Of course the union isn't the employer. They're a group whose purpose is to represent, and negotiate on behalf of, the *EMPLOYEES*. So, why shouldn't the employees be allowed to negotiate certain aspects of others employment agreements through a group founded for that very purpose?
A contract about the terms of hiring workers is NOT negotiated by the one hired, if it's negotiated by a union. So why should I be bound by a contract that I had no say in to participate in an organization even if it promotes positions that I disagree with?
Freedom of association is an important right, and it's a good reason to allow unions. Similarly, the right of petition is important.
But does that mean that a union should be able to effectively block the right of all employees to be unassociated (without which freedom of association is meaningless), or to force them to petition in favor of a cause which they don't support?
And don't say "You're free to go work elsewhere, so your freedoms aren't infringed."
Would you say that if someone were fired or not hired for being an atheist? What the immediate cause was that the other employees didn't like that attribute?