no one can yell scam at someone if the risk are advertised.
You're wrong. Even if they tell you it is a ponzi 'game' the end result will be the same, the scammer walks away with a 'profit' he steals from the victims he has conned into sending him money.
A Ponzi scheme, by its very nature, will collapse and leave the majority of its victims at a loss. The only person guaranteed not to be out of pocket from these scams are the operators who get to choose when and how they steal from their users.
Users who participate knowing the truth about these scams hoping to be in the lucky few who might, *might*, get back more than they put in, are basing that hope on their being able to steal from later 'investors', too. Not all of these later investors will realise what they are sending money to, so you can't honestly claim that everybody who takes part knows what they are getting involved in.