So, since your argument is so solid, and you understand basic business so well, could you please tell us how many modders and competitive products Valve has sued out of existence in relation to Team Fortress? They own the Team Fortress IP, they hired the creators of the original mode, and they subsequently released Team Fortress 2 as well.
From 1999 until now, there have been a gazillion different mods on a variety of different engines, for a bunch of different games, using the Fortress name openly to signify the sort of gameplay the mod entails. How many of these people were sued by Valve, the holders of the Team Fortress trademark?
Surely Valve hired the creators of Team Fortress not because they wanted them to develop a sequel they would effectively give away for free right? No way. They definitely hired them to own the rights to the game, profit from it and sue others, not sit back and see your market share drop while some other guy is doing it for free. Right?