The First Amendment only says that the Gov't cannot censor you. Your arrangement with your employer is a private agreement. You bad mouth him, he can fire you for it. He cannot stop you from stating your opinions, but he doesn't have to continue employing you either.
But, as I understand it, discussing anything union related is allowable on company time. (IMHO, if an employee is spending so much time debating union rules and griping about the boss that they don't get their work done, that isn't right either. I want things to be fair on both sides. I am pro-union, but sometimes unions take things too far, and every job has that one or two guys that always gripe and never work. That isn't good for the employer, the other workers, or the customers.)
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