...How is this "unconstitutional"? Not everything that is illegal is unconstitutional. I am also skeptical that this is illegal, but considering the jurisdiction may have many applicable laws, it's hard to say that it is or isn't illegal without significant research. But surely it has nothing to do with constitutional law.
Similar pieces of other contracts are valid and common and are called "non-disparagement clauses". I don't see how this differs.
You're quite right. I hadn't realised that the constitution relates to actions between the government and the people only. Reading HN comments is always educational! I suppose it's more correct to say that it isn't in the spirit of the First Amendment, or free-speech in general.
Similar pieces of other contracts are valid and common and are called "non-disparagement clauses". I don't see how this differs.