Copyright is automatically and exclusively on the initial creator's side. What we got is an explicit license (read it) to use ironclads work in exchange for money.
It`s ture that Ironclad own original copyright but it didn`t mean that you can modify and distribute any derivative works freely.
Yes it does, because Ironclad's, and any other developer's, release on modding and what you are allowed to adjust. Ironclad still owns the original copyright, and you recognized that, which means that their copyright release applies just as readily to anything falling under that copyright.
You can't use something in the public domain, tweak it, and then sell it for money. Why do you think the GNU Public License still exists? Because derivative works *MUST* fall under that very same license.
The worst-case (for my argument, at least) is that I cannot use any file created completely from scratch from a mod, but I can still use any other file that was simply number-tweaking. Given that Distant Stars is a balance mod first and foremost, and 99% of its content are simply tweaked 'core' files, they can't even begin to TRY to claim 'derivative work'.
If 'derivative work' were as liquid as you seem to think, there'd be little claim for mods being shut down by Fox for being 'based on' the Aliens universe, or a Star Trek mod being C&D'd, as you could claim Derivative, or at best, Fair Use, neither of which has been successfully fought under.
Finally, there's history. No mod has, to my knowledge, ever been able to use 'derivative work' to claim ownership over its own content. To do so would allow them to even sell the mod themselves. On the other hand, we have two cases. Natural Selection 2 is going to be a mod for sale, but they also bought a license to the source engine, so they own their own creation completely. And Gary's Mod, which Valve is selling, not the mod's creators.
If you know otherwise on that last note, let me know. "Derivative work" also is extremely shaky because you own a LICENSE to USE the software, not the software itself, which means the terms of that license outweigh anything that you could normally do with, say, printed paper.