These venues can be distinguished from a Ponzi scheme because the participants are traders in a market, not investors in a company or managed financial product.
But retail traders can use the markets to make bets on overall economic growth. Buying a stake in hundreds or thousands of public companies is a clear bet, and you actually do own something in a legal sense.
With crypto trading, it's unclear what your bet is -- other than the continuation of the Ponzi scheme. You don't own anything, and the coin has no value as soon as people stop believing in it.
If retail investors understood that nature of crypto, I'd be fine with them gambling on it. But most of them are tricked into believing there is underlying value in the coin when there actually isn't (see also: NFTs).