> Should the state just prohibit all agreements between two parties unless the state's adjudicator decides they are exactly equal in "power" and permits it?

It's pretty simple. You can write whatever you want into a contract, but if you want to enforce an unreasonable term, you will lose in court and might be forced to remove the term from current and future contracts. That's how it works everywhere. The difference between legislations is just what is considered a reasonable term.