I think that there are a couple of issues here. Firstly, how are the staff in AB Towers to know the poster is for real and not just posting out of curiosity or to cause a stir? Secondly, if every moderator of every website reported to the police their suspicions of crimes, the police would be inundated.
I think there are caveats to this though. Remember the case where that chap committed suicide after being goaded by users of a forum? I think the moderators should have reported that. Secondly, if they found that a serious crime was being planned (an act of terrorism, murder, child abuse), they are probably under a duty to report it to NCIS.