What if it gets marked as "bad" the day after you bought it? And that's just one of the problems with the notion of taint: it's arbitrary and not transparent.

Let me put it this way: I don't do business with drug dealers, but I can't help it if the banknotes in my wallets have been used by drug dealers before. And I don't have to worry about it, because money is fungible. Without fungibility, we'd be back to barter, and even then, you'd have to worry if the sheep you received in exchange for a pig might have been stolen.