I believe the use of a contract and signatures in fact does literally make it legal, since that is the point of a contract. Everything depends on what you signed.
A contract cannot override law. A prime example is the minimum wage laws - it is illegal for an employer to pay less than the minimum wage, and an employee can't sign a contract to make it actually alright after all.
There are laws, and there are judgements which are the courts interpretation of the laws; a signed contract does not have the power to overturn either.
It is the job of HR & Legal depts to make sure a contract is legally viable, not mine, and I happen to know my rights. There are at least 5 illegal clauses in my contract, and I am certainly keeping them close by if I need to go to court against them one day.