I know Tilemill runs under the BSD license, but what about the "base data" that is included with it? I couldn't find any legal information in the guide. The best thing I could find is this:
"The Mapbox GeoData Library is a collection of free datasets that have been optimized to work well with TileMill. The library is browsable from the TileMill interface through the Add Layer dialog - click on the ‘browse’ button next to the file field, then click the ‘Mapbox’ button with the cloud icon.
All text within the data has been converted to UTF-8 which is the preferred encoding for working with data in TileMill.
All the data has been reprojected to Google Mercator projection, and is properly auto-detected as such by TileMill. All geometry data has also been cropped (where necessary) to fit within the Google Mercator square Earth bounding box of 180.0° east/west and 85.05112877980659° north/south.
All shapefiles have been indexed using Mapnik’s shapeindex tool. (The article Speed Optimization: Shapefile Indexes explains the benefits of this and how you can make sure your own shapefiles are indexed.)"
But there is no indication or links to licenses, etc. since I intend to use maps commercially (in YouTube videos, which have ads before them), this is quite important, because for commercial application the term "free" isn't enough.