To be fair, this could be said for any hierarchical structure. The first objective of anyone in power is to maintain power, and one way to do that is to quell dissent. On the other hand, you don't need to quell dissent, or even differing opinions, if your arguments are strong enough to stand on their own merits. Honestly, though, I don't think the arguments of union bosses--at least those in non-Right-To-Work states like California--are strong enough to stand on their own. That's why unions operate the way they do.
“The most radical revolutionary will become a conservative the day after the revolution.”
--Hannah Arendt, philosopher (1906-1975)
People want to stay in power. It's human nature.
Our 1st Amendment was passed to ensure that our political bosses can't stifle honest dissent. Our Founders, who had so recently deposed tyrannical rule, understood the need for such law. In American government, power can be fleeting. You may not have liked President Bush, but our system of government, and our American values, are ensuring a peaceful transition to a man who is his antithesis. In American governance, political power swings like a pendulum.
The same cannot be said of union governance.