Fallout 3 also has SecuROM but it's one of the most modder-friendly games around. That said, I think the SecuROM infestation is only part of the launcher rather than the game executable proper, so perhaps that's why.
I do not know about Fallout 3, but typically games you can mod look for the mods somewhere other than the installation directory which could be why SecuROM does not cause issues with mods for them. If that is the case then we would just need a way to tell the game to look elsewhere when loading stuff.
