Consent just means "agreement". The creators of the DAO (rather stupidly) agreed that "code is law", and the attackers agreed with the code. It's generally assumed in contract law that anyone agreeing to a contract understands it completely, including any loopholes. Abusing a loophole in a contract is perfectly legal and can be legally enforced if it works against the person who wrote the contract (it's his fault, after all). A person who discovers that a contract allows them to collect free money can't be blamed for agreeing to it; it's the responsibility of the contract's author to ensure the terms of the contract can't be turned against him.