I've always taken issue with this. Clauses are written so broadly that literally any creative work – a drawing you make for your kid, a poem you write for your spouse – is owned by The Company. You say this to a recruiter, and they say, "oh, don't worry, we won't enforce it." Screw that.
This was, is, and always will be a brilliantly shit illustration of how stupidly self defeating business people can be. Forgoing the universal positive of a staff self-training on their own time, because of the vague fear that one in a hundred might make some money.