New clause in communicators' contract?

Boks get about R160 million a year from Supersport for exclusive coverage, therefore Supersport are never really at liberty to knock and grill the product they spend so much on. Independent Newspapers also come in the picture because of the agreements reached by both parties on the reporting of Boks. In his new book Mark Keohane also reaveals that some Beeld rugby reporters consulted with Rudi Straeli (then Boks coach) on the players' ratings after each and every game.

But the point here is that Mark Keohane, as a former journalist, has used all this info he had access to and wrote a book about it.

Do you foresee a situation whereby communication managers sign contracts with clauses that say once they've left their jobs they're not allowed to talk about the "confidential" info of that particular organisation. What would be the legalities around that, especially as far as freedom of speech is concerned?