There's plenty of permissively-licensed tilesets out there. Dwarf Fortress really had some trouble because it was built on ASCII until a fan was kind enough to write a graphics library : https://github.com/Baughn/Dwarf-Fortress--libgraphics- . Writing an ASCII rendering engine was probably more work for a lesser product than going with a library to begin with would have been.
Also, Dwarf Fortress isn't done yet. It quite possibly ends with beautiful procedurally generated graphics (though I can't imagine that would happen with Tarn keeping it a solo operation.)
Not quite: Dwarf Fortress has always used OpenGL (http://www.bay12forums.com/smf/index.php?topic=28841.0) to render tiles that just happen to be ASCII characters onto the screen. The fan effort was to port it to SDL, making it much more efficient.
Also, Dwarf Fortress isn't done yet. It quite possibly ends with beautiful procedurally generated graphics (though I can't imagine that would happen with Tarn keeping it a solo operation.)