IANAL, but I get the impression that it may actually be in our contract because in many industries, it's much less contentious and has become standard that way. Take for instance, if I were a nuclear engineer. The equipment so expensive, and knowledge so specialized, that anything that one does in their free-time that is nuclear related is very clearly a derivative work, that these clauses are pretty much just an extension of don't steal our work and go to a competitor. And other things one may do, say woodworking, is so clearly hobby, and so unrelated that the issue would never stand up in court such that the issue is never debated.

But that's just been my understanding, I could be very mistaken.