How is this allowed?

[Mawlzy]

That's a good point, but questionnable when you see how unwilling SE is about doing anything about that. I guess there is no telling without an actual trial.

That said, text only can be innapropriate enough. When you advertise your PF as 18+ you know people will use the chat for unsafe things. I know it's not PF, but I remember a post from someone on the forum really pissed because someone DM'ed their 13 y.o. daughter (IIRC) to be a courtesan in a 18+ club, and they were pissed because their kid came ask them what a courtesan was. You might say it's not much, but it's still enough to fall under that article, and clearly enough to show that text only can still be inappriate.

I recall that thread and am obviously uncomfortable with minors receiving such an invitation.

Equally, unless incredibly sheltered, I imagine any 13 year-old on receiving an explanation of what a "courtesan" is would reply "oh, a prostitute!" and get back to playing the game. Alternatively they would simply Google it. As noted previously, they are just as likely to come across the term in (leve)quests.