Thats not what he is saying. You can say and do whatever you want, but the consequences are for you as well. From a certain professional level you can expect people to act accordingly. If you don't like something your boss does, fine, pick up your stuff and leave. If you gripe about it in public, also great, your choice, but it does not come across as very professional, on that I agree with Titus.
It has nothing to do with professionalism and everything to do with hierarchy and management stamping out dissent. People are afraid for their jobs, they don't like anybody rocking the boat. 101 reasons being self employed is great.