But at least I know if I'm ok with it, so it can't be exploitation. If I wasn't ok with it, I could change it. The hardworker doesn't even know he's getting paid less (because I made sure of it in contract). See the problem?

Being paid less than someone else does not imply exploitation.

Contractually preventing a worker from talking to other employers, hindering price discovery, may be exploitation, but one would know about it before entering into the contract and would only do so if they are okay with it. So long as the worker is able to talk to other employers, then they can learn just as much about the market as you are able to learn when talking to other workers.

That said, even if said contract was signed, it is unlikely a court would uphold such an obligation, especially when there is nothing offered in kind. They generally do not take kindly to contracts that completely remove one's ability to stably earn a living. The state doesn't want you to become its problem.