Because, sometimes, an 18+ event isn't just an ERP thing. Sometimes it's just that adults would rather RP with other adults for comfort, sometimes it's due to more mature (non-sexual) themes being written about they want to have taken with the proper amount of gravitas and maturity. Conflating it all as ERP is disingenuous, and hurts those groups looking for people -- I mean, FC's can recruit for 18+ and that's never been a purported issue before. If it's for a term like "dalliance" which is used in the context of the game itself, and is a word with multiple meanings, then the level of word-policing one has to do just to post a make-believe event in the game becomes outside the bounds of reasonable and will then be applied to otherwise innocent PF's just because of a technicality. And all of that compounds onto itself and hurts the communities. It also just opens the door for people with bad blood between them to harass each other by knowingly taking advantage of the way GM's operate in this game.
And, anyway, you won't fix the problem you're on about. Explicit, sexually-charged, events will still happen. So all that's being accomplished is hurting people who would use the tag, use the word, for otherwise innocuous purposes all because a number of people used it to advertise for whatever strip club they're doing (who will, again, continue to advertise for said strip club after the fact, anyway, just with a Family Friendly G-Rated Ad).
Hurting community generated content is, to me, the last thing people should really want. Sure, punish the people who're stepping over the bounds of reasonable -- but the description of the PF ad in question in the first post really... isn't worth all this.
Which, my point being: it'll just hurt more people while solving no problems, it'll hurt the overall in-game community and lead to more isolation and distrust amongst others that's already a hallmark feature of this MMO; and expecting better discretion and more clear rulings from GM's so there isn't a thread 18 pages long where people don't really know why someone was infracted but can only make a contextual guess on the matter isn't a ridiculous ask.