Ignorance, as far as I'm aware, is not an acceptable legal defence (as cited by many a judge to people who neglect to pay their taxes).
Be careful not to confuse criminal law with contracts. It is fundamental to the existence of a contract that both sides understand it. That is one of the few points of law that is pretty much a universal constant, whatever jurisdiction you're in.
The way this was explained to me, and a phrase that I think is very powerful, is that for a contract to be valid there must be a "Meeting of the minds"
(IIRC there must also be two parties (otherwise it is a deed not a contract) and the parties must have "capacity" i.e. the potential to fulfill their obligations)
IANAL - probably painfully obvious to anyone who is.
Law is like code (and magic) - it is a framework that gives real world power to abstract language - maybe this is why HN is so obsessed with legal chitchat
And the legality of this isn't fully established; in fact, last I heard, it was being somewhat successfully contested. But there's no sweeping word on the matter yet: you could probably take a ToS violation to court and have it set precedence.
Be careful not to confuse criminal law with contracts. It is fundamental to the existence of a contract that both sides understand it. That is one of the few points of law that is pretty much a universal constant, whatever jurisdiction you're in.