You say you must opt out in same states. In a few nations, you must opt out. I don’t believe that is the case in any state. Please cite one, I could be wrong, but I’d be surprised.
It took a while but finally found the article I had read. You are right, the U.S. is completely an opt-in country. It was in Israel that they had opt-in legislation pending.
I remember looking it up because someone from California told me that ‘organ donor’ was on her driver’s license and she hadn’t opted in. She must have agreed to it.
Countries vary tremendously in terms of definition of brain dead and associated legislation. There are a couple of summaries out there on the web.