Heyas,
There is case history where if it is found that the union (or employer) is interfering with free speech, there are horrific penalties and damages, not only to the union or corporation, but the individuals responsible are also held personally liable.
Unions cannot delete so much as a forum message without being EXTREMELY careful of this. Your message pretty much has to explicitly violate the terms of service (no calling names, personal threats, etc). As mentioned above, if they do, the responsibility falls on the union, communication chairman, right down to the forum administrator who actually deleted the message, and they can all be held collectively and individually liable.
If you can prove that they have interfered with your freedom of speech, you make bank.
Look up the Labor Management Reporting and Disclosure Act (LMRDA)
Nu