PoW captchas are essentially cryptocurrency miners for the server. when the server throws the result away, instead of using it to pay for the server or whatever, it's PoWaste

it wouldn't be quite as bad as if the server actually took the PoW/mined currency as payment, ideally also offering alternate means of anonymous proof of payment for access (GNU Taler, browserless mining, whatever) to bypass the on-browser PoW