The only (other) time I have heard of this being enforced is in a totally unrelated venue. The rumor's behind why Tim Burton chose Disney/Mirmax to release "A Nightmare Before Christmas" is that he came up with the story idea while he was an animator at Disney.
This is also supposedly the reason why HP had first crack at the original Apple, but turned it down. Wozniac had a clause in his contract.
This is the standard clause for professional creative development, and is one reason why at a certain level most creative professionals become independant contractors.
