It's pretty surprising that it's so damn easy to hack all these exchanges.
Why do they always hold so much money in hot wallets? It's stupid! ....
also, this one mentions FOUR hot wallets.... LOL?
It's not that uncommon for exchanges to use several hot wallets. It largely depends on the frequency of people cashing out their coins. If people aren't cashing out their coins frequently, it means that less coins need to sit in their hot wallet. But if the opposite is the case, where people constantly withdraw large numbers of coins, they need to stack their hot wallets in order to process everything without any delays - for people's convenience. Main reason for exchanges to operate several hot wallets is to avoid putting everything into one basket. It has all to do with security, which in this case didn't stand a chance as the hacker gained access to all four wallets.
Not uncommon? Maybe.
Stupid? That's for sure.
Why would they need 4 hot wallets if they could have just 1 with a single person responsible for it?
1 Employee sitting in front of a clean machine with only security programs and network monitoring that would immediately show someone connecting from the outside and a single wallet that only you know the pass to.
All trading is done in the API without the access to actual coins.
Hot wallet with a limit of, say 500BTC. Once that is reached and there are no big withdrawals pending, you send half of it to your cold wallet.
This makes your hot wallet handle small payment only and you got only 1 person that manages it. Really handling a couple hundred payments a day is nothing for a full time employee, especially in Asia.
Big numbers are handled by the cold wallet machine that goes online only once a day at random hours for a short while to process the transactions. This is handled by the owners of the exchange with only 2 people having access.
Banks have big vaults guarding the money and bitcoin businesses are so careless.