Consent is not a simple matter when it comes to medical ethics.
In many cases, active consent is dismissed in favour of presumed consent where it is considered to be in the best medical interests of the patient, or in this case, patients multiple.
It's a debate that could be had endlessly and never come to a true resolution - is it better to assume consent and save another life or lives, or does the principle of individual consent override all? Which is more ethical?
I don't wish to derail the thread, because we could get into numerous detailed "what if" scenarios, but suffice to say that there is no one "right" answer, we have to have some kind of system (which is always going to be considered unethical by some, regardless of what it is), and presently it seems to be felt that the majority would consent to organ donation and so here we are.