if the code's open source they couldn't prove that it was you who was kept releasing it.
Open source code has maintainers, who can sent CnD notices.
It doesn't have to have - if Duke released the source code, he could keep releasing versions on a torrent site and claim ignorance that he doesn't know who's releasing it as it's open source (not that I'm condoning breaking the law or anything).
98% of all these copyright things are usually because the lawyers HAVE to make a copyright case (as by not defending it now, they set a precedent for later when they need to) if you're not charging anything for it and it stays moderately low key (Bay12,/tg/, whatever) I can't imagine they'd actually waste money trying get you.
Well, no, they don't have to. They can set up rules where you can use their stuff if (for expample) it's free and you wrote them an e-mail "asking" to do it. (That's what LucasFilm did.) It just ends up being cheaper to pursue copyright agressively, then to enforce rules. Shame that.
It's
CERTAINLY not cheaper to pursue copyright aggressively, the thing is that they need to do it to protect their copyright as otherwise if someone does properly rip them off later, they'll have a harder time winning in court later, as they didn't bother to uphold their copyright last time (see the whole 'Sins of a Solar Empire: Rebellion' case for more on that). As in the SoaSE case, the 'rebellion' holders just made a complaint even though they knew it had no chance of winning. Just like the Candy Crush Saga thing did the same. It doesn't make much sense, but unfortunately that's the way it is.
Fair use and all that is fine, but there's a difference between that and actively ripping off all their assets. Someone could easily think that CM is a GW official game, and even with notices and things, it'd still be far too similar.