The NCAA assembled a team of medical and disaster response experts this week to make recommendations. They basically said at this point they are not recommending the Div 1 tournaments be played behind closed doors. That being said, if leaders in the cities where the tournament is being played decide to declare a health emergency like they did in Austin (South by Southwest, a citywide festival that attracts hundreds of thousands of people, was cancelled), there's not much the NCAA can do.
The issue comes down to the fact that COVID 19 has a hospitalization rate of 30%, with 5% needing ICU care. If the virus spreads through the general public, our hospital infrastructure will begin to crack (best case scenario). Then you have people with other emergencies, some potentially life threatening, that won't be able to receive care. It snowballs from there.