Ponzi schemes are per definition a scam and puts legitimate sites in this section in a bad light. Hopefully, the mods here can take action and at least move these sites into another section.
Why are these a scam?
They are only a scam when they are advertised as something they arent. E.g. a guaranteed 150% return on your investment.
If people are fully aware of what they are getting involved with - i.e. it could collapse at any time, just like a betting site if it had to make massive payouts, I dont see any problem with it.
It's very much a game of luck. (Will you get your payout before it goes bust.)
I dont see ponzis, if sold as a ponzi, a scam. I think too many people associate ponzis with scams due to too many "owners" running away with the wallet.