[CPLX]

They can’t prevent you from doing anything, but you can agree not to and agree to consequences if you don’t.

Whether this is ethical or not is another (and important) question but that’s the legal framework to it.

[pannSun]

There are plenty of limits on what contracts may contain*, so the "it's legal if both parties sign the dotted line" is, for now, fiction. It's also what every entity able to compel people to agree to its contracts is lobbying for.

*For example, in right-to-work states, employers can't agree to hire only union labor. Strange that that is prohibited, but an omerta imposed on workers is not.