It's in a secret section of the constitution on employment law, that overrides the first amendment.
The first amendment (and all the rest of it) has nothing to do with non-governmental, private interactions.
Of course it does, when you're talking about the government regulating non-governmental, private speech, such as this mythical law against employers saying truthfully that you were fired for cause.
Fair point. I suppose I'm really not sure.
That's why companies are free to have this as a company policy.
The First Amendment prevents this policy from being a law.