They believe the global pandemic justifies their action. (They give their legal and moral justification here.)
The National Emergency Library is not just medical books. It is everything they have. For the duration of the emergency you can download all kinds of amazing things. Here is a list of their numismatic holdings by decade. It includes some very rare auction catalogs and club journals.
I encourage everyone looking for an at-home project to consider finishing your own numismatic works and prepare them for publication. Stay home. Use this library for research during the pandemic.