To try and answer that one question: It depends on the behaviour.
Maybe they try to work around the barrier by wearing a ballgown. Maybe they believe that it's so very unlikely that someone wears something with "gown" in the name, especially on beach sims, that the word "gown" is not on the filter list, thus you won't be expelled for wearing one. The major sign for this is if they've actually tried to breach the barrier in the gown. A minor sign would be female avatars with BoM mesh bodies wearing old layer-plus-flexi-skirt ballgowns designed in the 2000s for the system body rather than the SL imports hanging in every more recent freebie store. The idea behind that would be that it's even less likely that these are on the filter list.
Or maybe it is a combination of laziness and lack of common sense. The kind of female avatar that always wears the same clothes, regardless of the situation. You know, the kind of female avatar that'll soon be seen at least by the dozen running around snow-covered Christmas sims in sandals with 10cm spike heels and sleeveless micro-mini dresses without tights. The female counterpart to the guy who goes to the nude beach in a black leather jacket. They always wear that, they love wearing that, and they can't be bothered to change.
Why a ballgown? Because these avatars refuse to wear these tiny club dresses that seem to make up 50% of all women's fashion SL imports. They don't like super-tight jeans, super-tight leggings and ripped jeans hot pants either. They have yet to find the four SL import boxes with long skirts/dresses that aren't ballgowns. And they will either probably never discover the rare sources for different clothes or, if they do, dislike them, too. So ballgowns as daily wear it is.
This is rather unlikely, though. Women will rarely only make one outfit for their avatars. They're more likely to use them as dress-up dolls, so they would certainly have at least one outfit that's rather fit for the beach. Those who don't like "dressing up" and wear the exact self-same outfit for years are mostly men and a great deal of them. If you see a female avatar always wearing the same clothes everywhere, especially if it's a rather low-effort outfit, it's likely that there's a real-life man behind the avatar.
I could also suspect that they did actually just come from a ball-like event and decided to go check out your sim together because they needed something to do after the ball. But did any such event occur recently, an event that justifies or even requires wearing a ballgown? Besides, if such an event takes place, how many of the women will actually wear a ballgown? 30% 40% maybe? That makes it even more unlikely that a 100% ballgown-wearing group comes from such an event.
But ignorance has always played a key role in occurrences on sims that allow, encourage or even require nudity. Typical situation on Stark Islands, especially past 13:00 grid time: An avatar arrives at a Stark party for the first time. Stays there and waits for everyone's clothes to rez. Not easy with 10-20 avatars around. Notices that it takes longer than usual for attachments on certain avatars to rez. Realises that there aren't, in fact, any attachments that'd have to rez on these avatars, and that these avatars are actually naked. In public. At a party, even, amongst other avatars. Leaves without saying a word, deeply disturbed and disgusted.
The Adult rating was originally created as a warning for prudish visitors that they may encounter fully nude avatars, if not even public sexual activity in a given place. It may still work as such in Second Life, but it either does no longer in OpenSim or has never done so in the first place. People have gotten used to Adult only meaning no kids allowed, even if it's on a perfectly clean sim that'd get away with a General rating, but the sim owner hates kids. And lots of grids actually allow for weakening the Adult rating to the point where all its liberties over Moderate or even General are taken away, and only the "no children" limitation is left. No wonder many people don't expect Adult things on Adult sims anymore and will protest against them.
However, it's even worse than that. You can plaster words like "nude" or "naked" all over your sim's advertisements and its name. People either can't be bothered to read that, even if it's in the sim's name. Or they believe, it's only a name or something to spice up the description, literally nobody ever obeys dress codes in OpenSim anyway, and especially, nobody really ever gets naked in public in OpenSim, no matter where. Or both. And then you've got prudish and up-tight people entering a no-clothes-allowed-whatsoever nude beach (complete with signs on all entrances in-world), fully dressed in their streetwear, of course, and then freaking out over the sight of one naked avatar.
I still expect someone to sooner or later downrate a nude sim and write a scathing review in which they loudly complain about the naked avatars. Either they'd clearly show that they haven't even read the sim's name (evident if they demand the sim owners do something against all the naked avatars), or they'd be surprised that people actually DO go naked there.
I tend to believe that the only way of keeping people out who are both prudish and dumb is by naming a sim something with "BDSM" in it. But then you'll have to deliver, otherwise the BDSM fetishists will complain that the sim has got nothing to do with BDSM. This may be the reason why not exactly few Adult sims are BDSM-themed: Adult rating and Adult category in combination with "nude", "naked" and even "sex" in the name don't drive prudes away because they don't think any of this actually occurs on the sim.
So I believe the nudity barrier at Amoa serves well against those ignorants who don't even read sim names before teleporting there or who think it's just a name, and neither the name nor the description (if they've read it, that is) are backed by anything. They'd otherwise happily stride around the entire sim in streetwear, completely unaware that this IS actually a nude sim. Getting teleported back to the landing repeatedly when trying to enter the sim should get them to a) read the sim's name, b) read its description and/or the huge sign at the landing, c) accept it as a matter of fact and d) act accordingly.
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