Earlier quoted context omitted.
I live in Nevada County, which apparently has a very low infection rate (34 cases, none in the last week) but if you open this county, what prevents folks from other countries from flooding in and bringing more infection? This county was previous a tourist mecca so it would be especially inviting. But any county that was open would be an appealing destination for the closed counties.
I see this as an argument for a county-by-county approach in which the specific circumstances of the county are taken into account.
I don't see why any "deemed safe" county wouldn't be overrun by visitors from unsafe counties, making the county unsafe again. Maybe you could find a county so remote it wouldn't matter but whether those extremely isolated places are opened up now or not probably doesn't matter either way. I doubt they're much effected by this situation regardless.