"If I'm your president," Joe Biden promised at the Democratic National Convention last summer, "on day one we'll have a national mask mandate." Now that Biden is the president, his policy has changed from a general requirement to an executive order that applies only on federal property and interstate transportation. While supporters of a nationwide face mask mandate to combat COVID-19 may be disappointed, constitutionalists should be pleased. The switch is a hopeful sign that Biden acknowledges limits to presidential power, even during a public health emergency. Until he backtracked in September, Biden had been promising that he would "do...