Brain activity is not correlated to alive or dead. We don't have any idea where consciousness comes from, much less what constitutes recoverable vs non-recoverable brain damage. There are cases of people who were fully awake and alert showing 0 activity and vice versa. Terry Schiavo was not only alert and awake, she was fully able to interact with everyone around her.
It's literally just some shit they made up to justify taking living organs from living people who can't defend themselves.
But you are right, the hospitals in America do harvest organs from the living.
It is possible for your brain to have no activity but your heart still beats for a little bit.
It's literally just some shit they made up to justify taking living organs from living people who can't defend themselves.