If I am sure that someone's cryptocurrency does not meet the fixed standards
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 wallet 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 transactions, and even then, you'd have to worry if the sheep you received in exchange for a pig might have been stolen.
*This is actually quite likely, 90% of all banknotes contain traces of cocaine.
I understand what you mean. There is no completely pure money or coin. I will review every asset I acquire and then transfer it to the exchange. If the other party's wallet enters unclean coins the next day or at other times, it will not affect me because I acquired it before that. So the AML assessment I need needs to be instant, with almost no delay, or within 1-2 hours.