IANAL, so I really do not know. But what I do know is that a lot of people do not have the resources to fight a company in court over it. I had an experience where I was unfairly terminated, and the lawyer I consulted with said I could take it to court, but it would most likely cost more than I’d receive. I’m sure that is a common sentiment in our (US) legal system as a whole.

Non-competes aren’t enforceable, but proprietary “work-for-hire” clauses might be. I think the difference is that a company can’t stop you from working for a competitor, but they can force you to sign away your rights to anything you create while employed there. I have worked for a couple of big-box consulting companies, and they have you list anything you created before joining them, so any new creations are automatically theirs.