Well a return is not promised so it's not a true Ponzi, since you tell people they can lose all their money. In a true Ponzi scheme the nature of the scam is hidden from investors. But you have called it a Ponzi game, so the meaning is a bit blurred. Whether it is legal or not I am unsure but it's not exactly high on the moral chart   It definitely goes against the purported aim of most of the cryptocurrency community although I realize that is a complete joke  
http://legal-dictionary.thefreedictionary.com/Ponzi+Scheme