Either way, Ponzi schemes can be very profitable. I don't think there is anything unethical about gambling with a Ponzi scheme.
Do you find anything unethical about early adopters defending Ponzi scams so that more people buy in (so that they can get out before it disappears)?
Do you find anything unethical about running Ponzi schemes, and trying to get as many people to buy in before you skip town?
As long as people understand the risks, then no. There is nothing unethical about it. Just like there's nothing unethical about encouraging people to play slot machines.