You seem to be thinking of a ponzi scheme as just another financial instrument when they are not.

The negative reaction to this is because there is no such thing as a fair ponzi scheme. They are inherently unfair, dangerous instruments created with the sole function being to enrich certain people at the expense of others. For that reason they are illegal in the US.

They're very easy to program in Solidity because it's easy to shuffle value between a bunch of people. What's not easy is creating a financial instrument that can be beneficial to multiple participants and uses risk as an incentive rather than just a pure gamble.

The Dai stablecoin is one example, where individuals can withdraw money (dai) with ether as collateral. They can use that money however they like, and a fee is paid to the system and their collateral can be auctioned off if the value of ether changes such that it doesn't collateralize their withdrawal.

Multiple participants and multiple beneficiaries.