Well you have a poor understanding of the definition of a Ponzi scheme. ICOs do not make any promise of a return on investment or paid dividends, the way a Ponzi operator would. Coin makers do not use the income from new investors to pay dividends to older investors, the way a Ponzi operator would.

You are buying the currency, once you receive it the contract with the coin's creators is fulfilled, you are not an investor, you do not own any shares of any company. It is a one-way transaction.

Perhaps you meant pyramid scheme, of which ICOs are also not an example.