That is not a Ponzi scheme. A Ponzi scheme is giving someone payouts on their investment funded solely from people putting the money in, while fraudulently telling them that the money is coming from real investment. You can't have a Ponzi scheme when on average both sides of the transaction are (generally) aware of the situation and aware of how the value is flowing, because you lack the fraudulent aspect. Also, for any given transaction, value is flowing instantaneously; when someone sells their bitcoin, they aren't getting paid in funds being fraudulently kept aside from somebody else's previous purchases. It's an instant commodity exchange.
Again, in a nutshell, the reason why this doesn't fall afoul of laws against Ponzi schemes is that it simply isn't one.