I was reading this GIS.SE question about whether TileMill will be replaced by Mapbox Studio. I got worried about the future as a TileMill user, and the question suddenly came to me "isn't tilemill opensource?". Why can't we branch it if something goes wrong. To check, I went to their official website, and only found that [TileMill is]:
Powered by open source TileMill is built on a suite of open source libraries including Mapnik ...
Nothing is said about whether TileMill itself is open source. I went to the github site, and the license reads:
Copyright (c), MapBox All rights reserved.
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. ...
The rest of the text looks pretty much open-source. What about the "MapBox All rights reserved." part? Does that conflict with the open-source spirit?
I am not familiar with the legal terms. But I don't see any familiar license terms like GPL, LGPL, apache or BSD.
My question is:
is TileMill open source or not?