That doesn't mean that financial institutions that won't deal with US persons are legitimate. They don't deal with Bahama persons either, even though they are licensed to do so. That might get them in trouble with local regulators.
Of course it doesn't mean they're legitimate, but it's also not even a hint that they're dodgy. Likewise a lot of tax havens treat companies that trade locally differently, which is why many refuse to do business with citizens of their domicile. So once again, they may be dodgy but this isn't any indication about that.