It's not a crime to break a non-disclosure agreement. It's a civil matter. A person who breaks a NDA wouldn't be rounded up by the police and thrown in jail, they'd be sued into the ground by lawyers of the party that made them sign the NDA.
And you can absolutely sign away Constitutional rights when private agreements come into the frame. For example, Freedom of Speech. You give it up when you visit this very forum. Why? Because it's a privately owned forum. Your Freedom of Speech is only protected by the government in the PUBLIC. In private mediums of communication, the owner can decide what's allowed and what is not.
Your slave analogy is also flawed. It is criminally illegal to own a person. It is not illegal at all to tell someone "don't repeat anything I'm about to tell you". A person can agree to either in a contract. One would be easy to get thrown out due to the criminal activity involved, the other would be upheld because it's completely not criminal at all.
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