Employment Question – What's yours is ours?

Every so often on HN I see a question about those employment clauses where the company wants to claim everything you do in your personal life as being "theirs".

Is this unique to the tech industry (and if so why do we put up with it) or is this a general issue across the board?

I can see it potentially being there for artists/musicians though in a more direct manner - where a singer can't just start up a second singing career but if they embark on a photography or painting career the music company wouldn't claim to own it (perhaps?).