What other kind of source is there?
The script file used to generate the skybox. Do you do any research or do you take and accept things for granted? Have you read the GNU GPL?
2. Terragen's usage terms restrict to non-commercial only --> Only work done by the free version is restricted. Read the license carefully.
And who is going to buy the tool to edit the same skyboxes?
4. Windows only product that's also not Free Software --> You do not distribute TerraGen, so it is irrelevant whether is is free or not.
I can not use TerraGen on Linux
I am not licensed to use TerraGen
I can not modify skyboxes without source that can
TerraGen isn't open source
TerraGen isn't free
TerraGen isn't Free
You might as well tell me to do all my 3d models in 3D Studio MAX or Maya and the sound effects in Adobe Audition, and music in FLStudio and Acid Pro too and then switch to Creative Commons BY-NC just to suit your agenda.
As far as I am concerned the topic is closed. I regret to say that most probably I am not going to contribute anything to the OA project. I have better things to do than waste time by arguing over licenses.
Suit yourself, but you've only really contributed nothing but suggestions of scavenging non-Free works and changing the core game anyway.
I should also note that the original game only had space skyboxes. The engine has a neat 'skydome' feature which does not require skyboxes, and is animated, and saves lots of disk space.