They can't restrict your speech in general,but at work they can and should do whatever they want. Practically, there is a difference. Unless they are willing to fire you, they can't stop you - get the difference ? For example, if you are so valuable that they are not willing to fire you, then they can't stop you. And if you are not, perhaps you should concentrate on work at work

I get your point now. Thanks. I was thinking about this in terms of the average employee. If the boss says you shouldn't talk about something and you want to keep your job, most people will begrudgingly do what they think they need to.