I'm going to answer honestly. There's a few different reasons I personally lurk occasionally and don't speak on campus, althought I can't speak for other mods. Like rose said, the TOU breakers come out more when we aren't visibly around, and we can catch people breaking rules. When I post on campus I often get into conversations with friends, have my wall spammed with questions, or get a lot of pm's. It becomes harder to watch campus and communicate to everyone at the same time - at the very least it seems I always offend someone for not saying hello or "ignoring" them. Sometimes I'm just only able to pop in for a few minutes at a time throughout the day, and I find it rude to post on campus and then vanish especially incase someone asks a question I don't end up answering or it appears like I'm not silencing people. Sometimes I simply don't have anything to say (shocking I know)