> But if that's the case for the US, what's stopping a thief to send money to an exchange like Bitstamp, buy BTC, withdraw it and then reverse the fiat payment in a similar matter?
Sending money to bitstamp requires an international wire transfer (SWIFT), those (usually) can't be reversed.
Its ACH payments which get reversed so easily (ACH is a domestic network, all in-country). That's why TradeHill shut down after a fraudulent $20k Dwolla payment, all Dwolla deposits/withdrawals are through ACH. And that's why Coinbase has to be so careful with which customers they allow to purchase bitcoins, all Coinbase deposits/withdrawals are ACH.
> And while it's true that on Bitalo you have to handle the fiat payments yourself, but it will be often faster to get BTC that way than to send it to an exchange, wait for them to process the deposit, and only then buy BTC.
Speed of a first purchase is one thing. But I'd guess that the main reason people keep funds on an exchange is because they want to day-trade, pick up cheap coins with a lucky limit order during a flash crash, or have the option to panic sell in a split second (not even a one hour wait for 6 confirmations). That kind of trading is only possible on an exchange with a limit order book, not an OTC marketplace.