So here's your choice: Polluted waters that NO ONE can use for swimming, fishing or whatever but those 1 in a thousand people who would otherwise eat these oysters will be safe. Or we have less polluted (and eventually clean) waters for everyone and polluted oysters that nobody can eat. Which would you pick?
I understand what the fear is. They fear that someone will either unwittingly take them and sell them or will knowingly do so (evil bastard) and someone will get sick and sue and blah blah blah.
Look, life has risks. We can mitigate them but not eliminate them. Just put these oysters in an area that makes them difficult to cultivate or cost prohibitive to do so. We need clean water more than we need fear bad oysters.