Opposite to what? When you deposit into an exchange you are not given fiat value into your exchange balance, but you are given only RIGHTS to trade onto their exchange with their offered value of your deposited asset.
You have to consider that, like crypto hot wallets, an exchange also have "fiat hot wallets" to address user fiat conversion requests on daily basis.
This means that part of the crypto coins must have been converted in to FIAT before the takedown. To allow users to withdraw if requested.
Exchanges usually convert a percentage of users' coins daily to feed FIAT hot and cold wallets.
Therefore, since the "FIAT hot and cold wallet" are presumably lost, a complete refund of all the assets is quite difficult, especially on the FIAT side.
But hey, I hope I'm wrong
You are right but those fiat hot wallets are insignificant to total value. Mayzus is not a bank so they have forwarded all deposits to BTC-e bank account. So only non settled withdrawal and deposit transactions are lost. There could also be situations when an exchange is completely out of required withdrawal hard currency and then they need to get or to borrow it from another source. But an exchange is not into obligation to keep exact amount of each currency as their client's exchange balances are.