Money first of all, coinLab is paying them for this. Also less risk/responsibility for mtgox, since coinLab will be managing the USD funds. An official partnership also enhances the professionalism of MtGox. The best part is even as this further cements their role as the biggest exchange, it does so in a way which distributes the risk. I'm not so worried now about a possible problem or crisis at mtgox, because even if the exchange goes down, my funds should be safe with coinLab.
So Coinlab will just assume every USD funds acct are from US/Canada? Thats stupid.
They must have known many ppl oversea also use USD funds.