I don't understand why this misinformation keeps being spread. Could you quote the specific law you're referring to? I have researched copyright laws on several occasions, and never heard of one that would agree with you.
You can use
this page for quoting. It's the _actual_ law.
If Firaxis used copyrighted material to create a piece of graphics, then they produced a derivative work, in which case the original copyright owner retains the copyright to the parts of his/her work in the end product. Firaxis can then claim copyright on their own contributions to the end product, but only on those. This would create a shared copyright on the modified picture, as per § 103 b. Note that Firaxis' part of the copyright could be declared void following § 103 a (because by not seeking permission, Firaxis violated the original author's exclusive right to prepare derivative works, as given in § 106 (2) ), but voiding Firaxis' contribution would be unlikely in this case, and not sought by any theoretical claimants anyway.
Shared copyright has some interesting ramifications, one of them being that the permission of _all_ copyright holders must be obtained to authorize a copy of that work. This means every time Firaxis/2K authorize a copy of their product (e.g. when someone buys Civ5), they do infringe on the copyright of the creators of the original works.
You're free to disagree, of course, but in that case I'd really be interested in the actual piece of legislation you're basing your claims on, because frankly I don't see any law that even remotely resembles them.