I mean, that's literally what it is -- functioning.
Is it a good society? I wouldn't say so. Would I want to live there? No. But just like there are plenty of societies on Earth today that the United States has no right to forcibly change just because we don't approve of their culture, the Federation has no right to overthrow Vaalian society just because they don't approve of their culture.
That's what the Prime Directive is all about: Recognizing that alien cultures who cannot threaten the Federation have right to be themselves, according to their own internal processes and politics, without interference from the Federation. Even if the Federation thinks their culture is dead wrong, the Federation has no right to force them to change. That's what sovereignty means.
I'm not entirely convinced the ancestors of the Vaalians were the ones who built Vaal in the first place. The whole situation makes way more sense if it's the remains of the intervention of some other offworld force ages ago. No prime directive issues in that case.