These fonts are not owned by Mapnik or Mapbox. According to https://www.microsoft.com/typography/fonts/font.aspx?FMID=1081,  `Arial Unicode MS Regular` is shipped with many Microsoft products.

You can read here how to integrate any font in Mapnik: 

https://github.com/mapnik/mapnik/wiki/UsingCustomFonts

http://weait.com/content/add-fonts-your-openstreetmap-server