They can't allow you to withdraw what they don't
"physically" have. I suppose that "physically" isn't the exact word for cryptocurrencies, but you get what I mean. They had coins stolen so even though we see some balances in our accounts, they can't be put into the blockchain because Cryptopia doesn't hold those coins anymore.
As stated here:
https://twitter.com/Cryptopia_NZ/status/1102776289650335744 they'll offer "rebates"... now what those "rebates" mean, I don't know.
I dont have ERC20 or ETH Balance on my Cryptopia account and majority of my funds is on USDT and its not affected by the hack.
They should be allowed to be withdrawn while the users with affected coins can wait for their so called rebates.