In my opinion, ponzis with express written rules (like all thousand ponzis I've seen pop up) are not scams in themselves, nor when they close down, as long as they do so in a way that abides by what they have stated to the public.
I agree with you that they aren't a scam if they operate how they claim they will operate. But I disagree that the majority of Ponzis do what they have stated to do. Most Ponzis claim to be able to double your money indefinitely (usually accompanied by some kind of lie about how they make the money from trading or whatever) and end up NOT doubling everyone's money.
Promising mathematically impossible things is what makes them scams in my opinion.
If they are clear up-front that some people will make a loss then I don't see how that can be called a scam. Alternatively, if they promise to double everyone's money, then realize after the fact that it's not sustainable and make up the shortfall out of their own pocket, then that too can't be called a scam. The vast majority of Ponzis of course do neither of these. They promise crazy returns and end up defaulting. That's what makes them scams.