Per the NEHGS's The Weekly Genealogist, with a link to the Open Culture website that featured the good news about the free maps online:
The NYPL announces the release with the explanation below:
The Lionel Pincus & Princess Firyal Map Division is very proud to announce the release of more than 20,000 cartographic works as high resolution downloads. We believe these maps have no known US copyright restrictions... .
From the NYPL Collection: