The issue is more cheats and bots that are the problem. Most things are just cosmetics or making designing houses easier and have no effect on revenue (in fact it might even increase it slightly as some RPers only stay subbed because of the modding community). Bots on the other hand cost the company money as they use stolen accounts or fraudulent credit cards.

Instead of trying to find where to draw the line, it's easier for them to say all third party programs that use or manipulates the game's data is against the ToS.