The thing loads of people forget is that if you don't try to protect your trademark, you lose it. One of my previous employers went through exactly this scenario, where AT&T tried to argue that they hadn't tried to protect the use of this company's trademark, and therefore had lost it. It was certainly eye-opening to hear that story. Anyway, it may be that they even like what the DoomRL dev is doing... But if they don't send a C&D letter, they risk losing their trademark on the name "Doom".