Keep in mind that there's also a matter of principle here. If PB hands out a free patch, JoWood has "won" - their game becomes a whole lot better, at the hands of PB, despite not paying anyone for it. I don't think PBs problem here is trying to helping it the fans, it's the fact that they would also help out JoWood a lot at the same time. In fact, they would increase JoWoods income greatly, and strengthen the Gothic brand for future JoWood Gothic games.
But now the fans are doing the same thing. Any 'net-savvy gamer takes the time to read what's going on in the community of a game. If someone reads that all the major bugs have been fixed by fans, a gamer is possibly more likely to buy the game. And the fans who fixed it didn't get paid. The have "helped" JoWood for free. But what they've really done is help a game that they love become more like what it should have been.
PB may very well look at it as if they are helping JoWood sell games if they fix the game they created. But they could just as easily look at it in the light that by fixing the game they are creating confidence and goodwill within their fan base... ensuring that those fans will buy their next project.
(Lets say that PB decides to fix Gothic 3 for free) - If I stood at the game store and looked at two games - one from JoWood, and one from PB... I would remember that PB took care of their fans and made their game the greatest it could be, and I would buy the PB game.
And I don't believe anything about legal issues. If PB comes to JoWood and says "We want to fix our game...no cost to you", JoWood is going to say "Ummm... Okay".
At this point that game has probably sold about 85% of what it's going to (if not higher) There aren't going to be that many more buyers. PB can only strengthen their reputation by fixing it, regardless if it puts a little extra cash in JoWood's hands or not. By fixing the game PB is putting future money into their hands, and ensuring their existence as a company.