As you say, the rights are often in opposition, and I would say that your proposed limit on speech in favor of association would actually require undoing limits on association that are arguably more fundamental. If a person can be fired for holding objectionable opinions, how far are we away from firing them because they talk about unionizing or because of their religion? Or will those things be on a list of "Things You're Allowed to Have an Opinion About"?

And I agree that the quoted sentence is too absolute, but I don't think the spirit is wrong, in the sense that behavior can be circumscribed by society in a way that that completely prevents it without it having been made illegal, and it's not convincing (to me, anyway) to suggest that the behavior isn't prohibited because it is, in fact, legal.