id Software makes its game engines free when their technology gets dated, but the artistic/creative content remains copyrighted. They do this by having a copyright "airlock" interface between the executable and the game data (WAD/PAK/PK3/whatever files). The lesson to learn from this conflict is that if you run an open-source project that encourages creative or artistic content add-ons that may remain proprietary, it's a good idea to establish a rigid copyright airlock between the free stuff and the creative stuff.