The US has limits on contracts as well, and courts can and will invalidate clauses or entire contracts for a variety of reasons. Read any of your terms and conditions, there is almost always a clause in there saying that if a clause is found to be unenforceable, you agree that the rest of the contract is still valid. The bar is a lot lower in the states, but it’s still there.

E.g. You can sign a contract to work for less than minimum wage, it will be entirely thrown out in court.