Anyone in the industry knows you can mod virtually any game to make something new. Take, for example, one of the most popular mods of all time: Counter-Strike.  Counter-Strike is a game that pits two extremely well-armed teams against each other -- terrorists and counter-terrorists -- on a map with a five minute time limit, during which the terrorists must either protect their "hostages" or plant a bomb in one of two designated areas.  The game it's modded from, Half Life, is about Gordon Freeman, a scientist at the Black Mesa Research Labs, who battles alien beings who have come through a space portal and are really angry about it.  If these sound even *remotely* similar, you should really up your Thorazine.

The point here is that you can mod lots of games into something radically different than the original.  Some publishers (like Half-Life's owner Valve) even encourage it to keep gamers interested in their core engines and rejuvenate their products for free.  Some publishers don't, like Take Two, who don't provide modding tools.  Why would the publisher be liable in either case for something they didn't do?