Hiccups, your last sentence hits the nail on the head. Not only can they dismiss the individual concerned, but if there was a proven 'libel' its quite likely they may sue the union and/or the individuals in charge of maintaining the BB as plaintiffs in that libel action. I hope they are fully indemnified!
There is often a dilemma with these websites whereby certain managers who are also nominally 'union members' retain full rights to access the union BB.
In my own company it is well known by all that the union BB content is downloaded on a daily basis by one such manager and passed 'upstairs' to senior management for review. This fact acts as a form of intimidation on the members, most of whom are afraid to make any kind of comment in case they raise their 'profile' on the management radar (our website does not allow anonymous posting).
In my view anyone gaining a management post ought to be required to resign from the union (a dog cannot have two masters) or at the very least should be blocked from any legal right to access the union BB. It won't necessarily stop the spying and tattle telling, but it can certainly make it less easy for the traitors in our midst.