Almost all of the closures have exceptions (implicit or explicit) for genuinely mission-critical travelers. Diplomats, medical staff, emergency contractors, airline employees, freight haulers, etc. Almost all of whom rely on commercial air travel. AFAIK, the point of the closures is to eliminate tourism and normal business travel. Somebody has to keep the air routes open for official and essential travel between continents. I'd imagine that United is doing this at least in consultation with various governments.