It's reasonable to expect your phone to not act like thermite randomly. Until they have 100% of sold devices accounts for, they should be accountable for producing something which malfunctions so spectacularly with potentially deadly consequences. I'm sure there are many people who still don't know about the recall (or who don't understand the actual risks and thus don't care). I'm sure that their notification system to urge owners to bring the device in for recall is not without bugs and didn't reach every last owner. I'm sure there are plenty of GN7s sitting in boxes.

It's not really the fault of a person to know that what they though was a normal phone is now considered contraband because the manufacturer goofed epically. Consider also all of the tech-illiterate who may have received one as a gift or whatever, and they don't know / care what model it is. It's not really fair IMO to fine them. I guess this is why they're checking all phones now.

I'm not sure of the exact mechanisms employed by them, but they likely need to work with every telco on which their phone can connect and go through all active IMEIs and ESNs to identify any still-active GN7s, likely at great expense to Samsung. They should also probably then blacklist those so the devices become unable to connect to the carrier, so any people still using them will actually bring them in for disposal.