The key factor here is that in the same 13+ rating for FF14, it also states

Online Interactions are not rated by the ESRB. The 13+ rating is based purely on the in-game content, not anything players might say or do.

Because quite simply, even in HelloKitty MeowMeow wonderland online you're going to find adults cussing, talking about adult topics, etc. Square Enix does little about the PFs simply because 18+ doesn't automatically mean naughty stuff. I know of a few 18+ Venues & RP guilds who have that tag purely because they deal in more darker themes & topics that are unsuited for younger people to hear.

That, and Square-Enix has a very "Don't show, don't tell" policy for these things. Long as any actual stuff is kept behind close doors and not having people ERPing in the middle of public in say chat, then Square doesn't really care.
Plus, as a previous poster said, these people are still paying customers, that quite often remain subbed when many other types of players unsub while waiting for the next major patch. It's easier for Square to do the innocent little whistle and look the other way while racking in money from their subscriptions while banning anyone who is idiotic enough to do public say chat ERP than it is to blanket ban this stuff and lose on a ton of free $$$.

Just take a look at many Social Media platforms that officially don't support NSFW content, but don't do anything about it unless a payment processor is bearing down on their back because even the NSFW users are still helping to pay their bills.