Maybe it gives them a feeling of power to be able to flout the contract they signed, and Apple surely has the right to terminate such employees if they're found out. One might even take a perverse view of the situation and suggest that Apple itself is sometimes dropping in a trial balloon into the public arena, just to see how Mac users might react. They can use this information to fuel their product designs. It is even possible that a deliberate leak is designed to create a feeling of excitement and expectation, which will drive more attention to the product announcement itself. Manipulating the press isn't just the province of a corporation.
Politicians are masters of the art.
Sure enough, when he appears before the cameras, the announcement is precisely what was expected. Makes one wonder whether he needs to seal a few lips or perhaps that's just the game, to drive interest in the event. Years ago, before I became a technology writer, I was a broadcast journalist. I covered town meetings, elections, the police blotter, school board sessions and so on. During that time, I got to talk to lots of government officials.