Pressure groups of all stripes see an opportunity in the coronavirus crisis, and that includes criminal-justice activists. Even as society locks down, localities across the country are unlocking jail cells and softening law enforcement to slow the spread of the disease. The risk is that public order starts to unravel at a time it is most needed.

New York City Mayor Bill de Blasio said Wednesday the city would identify for release “any inmates who need to be brought out because of either their own health conditions” or “because...