Adults of sound mind enter into all sorts of highly regulated agreements, starting from employment, to tenancy, to gym memberships, to even things like marriages [1].
The reason for why all those agreements are highly regulated isn't because they adults entering them are not of sound mind. The reason for it is that there is often a large imbalance of power between the two parties in the contract.
This kind of regulation has been the foundation of common law for nearly a millennium, and has been present in other legal systems for far longer than that.
[1] (As it turns out, you can't [2] marry someone with the stipulation that they can't divorce you, or with a fly-by-night, grossly inequitable pre-nup.
[2] Well, you can, but the judges will laugh your contract out of the room when you try to enforce it.