In many countries, you can exchange US$ for local currency, but not the other way around. That's by government decree. Yet the dollar is prized even though the decree says otherwise. In any case, there's much more than government decree factoring in what's bad money.
What's the counter example here supposed to be? Gresham's law says that people want to get rid of (spend) their bad money and get paid by the good money (US dollars, here). But government regulations mean they have to take the bad money and so therefore only the bad money gets used. You're describing the same situation where bad money gets used even though everyone would prefer to take USD.
In many countries, you can exchange US$ for local currency, but not the other way around. That's by government decree. Yet the dollar is prized even though the decree says otherwise. In any case, there's much more than government decree factoring in what's bad money.