From a legal standpoint, I don't disagree with anything that's been said. It does seem to me, though, that if the trip was purchased through a travel agency then the agency has an ethical obligation to advise to passenger of the requirements. What we don't know is whether there was any effort made.
In the long run yes, the passenger is fully responsible -- just as when I prepare your tax return you are 100% responsible for any mistakes I might make. Most passengers, unfortunately, aren't as cognizant as the denizens of these forums and they rely on the advice of their travel "partners".