Natural Selection source code released for free

Unknown Worlds, the studio responsible for Natural Selection 2 and the upcoming Subnautica, has released the source code of the original Natural Selection mod for Half-Life as a free download on Github. Despite being freely available, the code isn't technically open source -- some of the files in the package bear copyright license text stipulating that "contents may be used and/or copied only with the written permission of Charles G. Cleveland," which is sparking some confusion and consternation among GitHub users. Cleveland created the original Natural Selection mod for Half-Life in 2001, and the project was successful enough to spur Cleveland into forming Unknown Worlds Entertainment and releasing Natural Selection 2 as a standalone game in 2012. In 2013 the studio released the source code for the Decoda development environment used to build Natural Selection 2 as open source, allowing developers to muck around with the tools and assets used to create the game. Gamasutra has reached out to Cleveland for further clarification on how people may legally use the Natural Selection source code. UPDATE: Cleveland responded with the following statement: "It's not technically 'open source' as that is a very specific legal term. However, we want people to use the code to bring NS1 to new platforms. So I'm changing it to GPL as we speak (should be live shortly)." GPL, or General Public License, is a type of free software license that guarantees users the right to edit, share and use the software in question, but requires that any iteration of that software retains the same rights. It's the same sort of license that id placed on the freely available Doom 3 source code.