Ponzi schemes are fraud.
The key operating word (I believe) that you use is "BitCoin". Does Bitcoin nullify illegalities when it comes to fraud?
Absolutely not.
So long as something has value, that something can be applied to the overarching umbrella of fraud in the majority of jurisdictions. I know this is the case in Europe and even North America.
In sum, the legal stance on Bitcoin ponzi schemes would be that they are absolutely open to being prosecuted under the right circumstances.
The hard part (and why I highlighted the key operating word earlier) is that to prove a Bitcoin site is running a ponzi is substantially more difficult than proving a fiat-ponzi. It involves significantly more resources (I would imagine).
Thanks for the reply, is a Ponzi a fraud when it is clearly labelled as a Ponzi and sufficient rules\warnings in place? E.g. payouts are not guaranteed, only invest when you can afford to potentially lose etc?
That, in my own humble mind, makes it more of a risky investment scheme\gambling game than anything, as no deception is taking place.
Hopefully you understand where I am coming from, I think it is something, like many things BitCoin related, which are open to varying interpretation.