It’s not any different then ATC shutting down routes due to weather, even though there may not be any traffic in those routes. It’s called acting as “pilot in command”. How many VFR pilots do you know that ignore the “VFR not recommended “ from flight service? All of these shutdowns (except 9/11 of course) are nothing more than someone at the FAA or DOT covering their behinds worrying about any liability, instead of doing their jobs.