Isn't it just a matter of price though? If I was ready to accept a job for 100K, and someone said, "We'll own your non-working hours IP and pay you 300k" I would probably do it. Then I would work for them for a year, and do my own stuff for two. Companies try to make people sign it for free, but I view it as a negotiable item with a price, rather than a morality issue.