At the time when lobsters were fed to prisoners, they hadn’t yet figured out the boiling-them-alive trick, which is supposedly how you get lobster to taste good. I don’t know anything about oysters though.
Boiling lobster (and other things) alive is more about preventing food poisoning as bacteria appears quickly as soon as it's dead. You could kill a lobster right before cooking it (not boiled alive) and have the same flavor + reducing health risks.
