Forgive me again if I don't undesrstand again. Even if the code of somethign is reimplemented, doesn't graphics and suchlike still have an inherent copyright? I mean ,even though porting SC 2 to a new format changes (I assume) the code beyond recognition, all gfx and suchlike are still originals. Wouldn't this hold true for any title you port?
For clarification on this issue- remember that something can be copyrighted, and released under a copyright license such as the GPL or Creative Commons, which gives permission for specific uses of the copyrighted work.
So for a game like tremulous, the code and the content are copyrighted, but the code is GPL since it's based off the Quake 3 engine, and the content is Creative Commons Attribution-Sharealike. They've explicitly granted you permission to copy and make derivative works from the code and the media of the game, under certain conditions.
For UQM, the game is GPL and we have permission from TFB to distribute their copyrighted content with UQM, but not for people to make derivative works. So, you can download and play UQM, but you can't, in the strictest legal sense, take the ur-quan and put them into your new space game. (This is also partly a property rights issue, which I won't go into here.)
For a game like doom, duke nukem, or descent, the code is GPL'd but, as mentioned previously, the content is not under a free license. Thus the "bring your own WAD" requirement for the strictly legal distributions of 'Doom ports' and the like.